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/Frontend/ModuleInterfaceLoader.cpp

@@ -1690,6 +1690,12 @@ void InterfaceSubContextDelegateImpl::inheritOptionsForBuildingInterface(
     genericSubInvocation.setSDKPath(SearchPathOpts.getSDKPath().str());
   }
 
+  if (SearchPathOpts.PlatformAvailabilityInheritanceMapPath) {
+    GenericArgs.push_back("-platform-availability-inheritance-map-path");
+    GenericArgs.push_back(ArgSaver.save(*SearchPathOpts.PlatformAvailabilityInheritanceMapPath));
+    genericSubInvocation.setPlatformAvailabilityInheritanceMapPath(*SearchPathOpts.PlatformAvailabilityInheritanceMapPath);
+  }
+
   genericSubInvocation.getFrontendOptions().InputMode
       = FrontendOptions::ParseInputMode::SwiftModuleInterface;
   if (!SearchPathOpts.RuntimeResourcePath.empty()) {