[clangd] Crash when opening a C++ file with trailing backslash in filename

# Bug Report: clangd 21.1.0 crashes on Windows when opening a file with trailing backslash in filename

## Summary
clangd crashes immediately when opening a C++ file whose name ends with a trailing backslash (e.g., `app.cpp\`) in VS Code. The trailing backslash is **not visible** in the VS Code file explorer, making the issue difficult to diagnose. This crash is triggered during AST construction and results in a `STATUS_ACCESS_VIOLATION` (error code `3221225477`).

## Reproduction Steps
1. On Windows, open your target folder in VS Code.
2. In VS Code's file explorer: right-click → select "New File" → type the filename with a trailing backslash (e.g., `app.cpp\`) → press Enter.
3. VS Code will create the file **without any error prompts**, and the trailing backslash is **invisible** in VS Code's file list (the file shows as `app.cpp`).
4. Click to open the file in VS Code → clangd crashes immediately with the `write EPIPE` error and exits.

## Expected Behavior
- clangd should either:
1. Handle the invalid filename gracefully and report a clear error, or
2. Reject the file instead of crashing.

## Actual Behavior
- clangd crashes abruptly with a memory access violation (`STATUS_ACCESS_VIOLATION`).
- The server exits with code `3221225477`.
- VS Code shows errors like:
```
Connection to server got closed. Server will not be restarted.
Client Clang Language Server: connection to server is erroring. write EPIPE
```

## Environment
- **clangd version**: 21.1.0
- **VS Code version**: [1.108.2]
- **OS**: Windows [Windows 10 IoT LTSC 21H2]
- **Compiler**: `clang-cl.exe --driver-mode=cl`

## Crash Log Snippet
```
I[08:11:57.252] <-- textDocument/documentSymbol(1)
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Signalled during AST worker action: Build AST
Filename: w:\myGit\myProject\Frame\demo\d01_window\application.cpp\
Directory: W:/myGit/myProject/Frame/build
Command Line: W:\sdk\Clang_LLVM\bin\clang-cl.exe --driver-mode=cl /nologo -DEvent_EXPORTS [...]
Version: 1
[Error - 08:11:57] Client Clang Language Server: connection to server is erroring.
write EPIPE
Shutting down server.
[Error - 08:11:57] Server process exited with code 3221225477.
```

## Additional Context
- The trailing backslash in the filename is **invisible in VS Code's UI**, which makes the root cause hard to identify initially.
- Deleting the problematic file and restarting VS Code restores clangd to normal operation.
- This issue occurs consistently on multiple Windows machines with the same clangd version.