Introduce VisionOS Platform

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.

lib/Serialization/SerializedModuleLoader.cpp

@@ -852,6 +852,8 @@ getOSAndVersionForDiagnostics(const llvm::Triple &triple) {
       osName = swift::prettyPlatformString(PlatformKind::tvOS);
     } else if (triple.isiOS()) {
       osName = swift::prettyPlatformString(PlatformKind::iOS);
+    }  else if (triple.isXROS()) {
+      osName = swift::prettyPlatformString(PlatformKind::visionOS);
     } else {
       assert(!triple.isOSDarwin() && "unknown Apple OS");
       // Fallback to the LLVM triple name. This isn't great (it won't be