This change introduces a new compilation target platform to the Swift compiler - visionOS.
- Changes to the compiler build infrastrucuture to support building compiler-adjacent artifacts and test suites for the new target.
- Addition of the new platform kind definition.
- Support for the new platform in language constructs such as compile-time availability annotations or runtime OS version queries.
- Utilities to read out Darwin platform SDK info containing platform mapping data.
- Utilities to support re-mapping availability annotations from iOS to visionOS (e.g. 'updateIntroducedPlatformForFallback', 'updateDeprecatedPlatformForFallback', 'updateObsoletedPlatformForFallback').
- Additional tests exercising platform-specific availability handling and availability re-mapping fallback code-path.
- Changes to existing test suite to accomodate the new platform.
It turns out that on iOS builds theres variance on whether `OBJC_(META)CLASS` symbols get emitted & exported. What's important for TBDGen is that it matches what gets exposed in binaries and that is still upheld on iOS.
resolves: rdar://123811306
It turns out there are valid usecases for exposing a standalone
objc-metaclass symbols and tbd files must encode it correctly.
resolves: rdar://118499886
