fatalErrror trying to run tests on Windows

Replace this paragraph with a short description of the incorrect incorrect behavior. If this is a regression, please note the last version that the behavior was correct in addition to your current version.

**ArgumentParser version:** the `main` branch
**Swift version:**
Swift version 5.9-dev (LLVM ad32770d6738638, Swift 753d54576e043ca)
Target: aarch64-unknown-windows-msvc

### Checklist
- [x] If possible, I've reproduced the issue using the `main` branch of this package
- [x] I've searched for [existing GitHub issues](https://github.com/apple/swift-argument-parser/issues)

### Steps to Reproduce
Check it out on Windows. "swift test"

### Expected behavior
All tests pass.

### Actual behavior
```
Building for debugging...
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import ArgumentParserToolInfo
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.JSONEncoder
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.NSError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\UsageGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import ArgumentParserToolInfo
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.JSONEncoder
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.NSError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\UsageGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import ArgumentParserToolInfo
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.JSONEncoder
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.NSError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\UsageGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import ArgumentParserToolInfo
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.JSONEncoder
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.NSError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\UsageGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import ArgumentParserToolInfo
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.JSONEncoder
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.NSError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\UsageGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import ArgumentParserToolInfo
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.JSONEncoder
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.NSError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\UsageGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import ArgumentParserToolInfo
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.JSONEncoder
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.NSError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\UsageGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import ArgumentParserToolInfo
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.JSONEncoder
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.NSError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\UsageGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import ArgumentParserToolInfo
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.JSONEncoder
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.NSError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\UsageGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import ArgumentParserToolInfo
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.JSONEncoder
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.NSError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\UsageGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import ArgumentParserToolInfo
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\DumpHelpGenerator.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.JSONEncoder
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\MessageInfo.swift:13:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import class Foundation.NSError
^
C:\Users\dave\src\swift-argument-parser\Sources\ArgumentParser\Usage\UsageGenerator.swift:12:22: warning: using '@_implementationOnly' without enabling library evolution for 'ArgumentParser' may lead to instability during execution
@_implementationOnly import protocol Foundation.LocalizedError
^
error: emit-module command failed with exit code 1 (use -v to see invocation)
C:\Users\dave\src\swift-argument-parser\Examples\repeat\Repeat.swift:14:1: error: 'main' attribute cannot be used in a module that contains top-level code
@main
^
C:\Users\dave\src\swift-argument-parser\Examples\repeat\Repeat.swift:1:1: note: top-level code defined in this source file
//===----------------------------------------------------------*- swift -*-===//
^
C:\Users\dave\src\swift-argument-parser\Examples\repeat\Repeat.swift:1:1: note: pass '-parse-as-library' to compiler invocation if this is intentional
//===----------------------------------------------------------*- swift -*-===//
^
C:\Users\dave\src\swift-argument-parser\Examples\repeat\Repeat.swift:14:1: error: 'main' attribute cannot be used in a module that contains top-level code
@main
^
C:\Users\dave\src\swift-argument-parser\Examples\repeat\Repeat.swift:1:1: note: top-level code defined in this source file
//===----------------------------------------------------------*- swift -*-===//
^
C:\Users\dave\src\swift-argument-parser\Examples\repeat\Repeat.swift:1:1: note: pass '-parse-as-library' to compiler invocation if this is intentional
//===----------------------------------------------------------*- swift -*-===//
^
error: fatalError

error: fatalError
```

Start by checking out the main branch on Windows and running `swift test`, as described in the issue. Review the compiler output for the references to Sources/ArgumentParser/Usage/DumpHelpGenerator.swift, MessageInfo.swift, and UsageGenerator.swift. Done means the test suite completes successfully on the reported Windows target.

Written by the indexing model from the issue text.