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.
Previously, an extension decl was always considered exported (externally visible to module clients) as long as it extended an exported type. Extensions need to either contain some externally visible member (e.g. a public method) or implement a conformance to a public protcol, though, to actually be exported. Without this fix, the compiler incorrectly requires internal extensions to types that are always available at runtime to have declared availability which would be a nuisance for library authors.
As part of testing this change, I expanded the attr_inlinable_available.swift test case significantly and that prompted me to scrap the copy of the test specific to macCatalyst as it seemed like needing to keep the two tests in sync was going to be a liability going forward. I replaced the deleted test with a couple of more targeted tests that use `-dump-type-refinement-contexts` to verify the effect of `-target-min-inlining-version min` on the root refinement context. Again a macCatalyst version of the test is required because we don't have bots that are configured to make the macCatalyst runtime the "target" OS.
Resolves rdar://91382040
