Gradle should detect when Xcode is missing

When only the Xcode Command Line Tools are installed, presumably via `xcode-select --install` when the user most likely installed Homebrew or similar, Gradle fail with a cryptic error:
```
:compileTestSwift FAILED
xcrun: error: unable to lookup item 'PlatformPath' from command line tools installation
xcrun: error: unable to lookup item 'PlatformPath' in SDK '/'
```

In this case, `xcode-select -p` returns `/Library/Developer/CommandLineTools` vs `/Applications/Xcode.app/Contents/Developer` when Xcode is installed.

### Expected Behavior
Compiling Swift code without Xcode installed should give a meaningful error.

Reproduce the compileTestSwift failure with only the Xcode Command Line Tools installed, then inspect how the build checks xcode-select -p and handles the xcrun error. Done means compiling Swift without Xcode produces a meaningful error instead of the cryptic PlatformPath failure.

Written by the indexing model from the issue text.