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.
This involved adding a new substitution called %api_diff_data_dir that when
building against a host toolchain, looks in the host toolchain (next to swiftc)
rather than in the resource dir. The reason why I need to do this is this allows
me to perform a stdlib stage2 build without needing to build swift itself.
The only interesting changes here are that I had to add %api_diff_data_dir to
a bunch of normal/expected tests and also add %api_diff_data_dir's length to the
offsets in rdar31892850.swift.
