[ClangImporter] Update AvailAttr import for "macosx" to "macos" rename.

This resolves a number of failing tests caused by availability attributes
not getting imported for macOS.

(cherry picked from commit e0489d356a9683c444a118f490949aabd18adb0c)

Reapply this cherry-pick; it was accidentally reverted as part of
f4d68f6.

Author: devincoughlin

lib/ClangImporter/ClangImporter.cpp

@@ -1292,12 +1292,12 @@ ClangImporter::Implementation::Implementation(ASTContext &ctx,
   else if (ctx.LangOpts.Target.isMacOSX()) {
     if (!ctx.LangOpts.EnableAppExtensionRestrictions) {
       PlatformAvailabilityFilter =
-      [](StringRef Platform) { return Platform == "macosx"; };
+      [](StringRef Platform) { return Platform == "macos"; };
     } else {
       PlatformAvailabilityFilter =
       [](StringRef Platform) {
-        return Platform == "macosx" ||
-               Platform == "macosx_app_extension"; };
+        return Platform == "macos" ||
+               Platform == "macos_app_extension"; };
     }
     // Anything deprecated in OSX 10.9.x and earlier is unavailable in Swift.
     DeprecatedAsUnavailableFilter =

lib/ClangImporter/ImportDecl.cpp

@@ -6187,11 +6187,11 @@ void ClangImporter::Implementation::importAttributes(
       auto platformK =
         llvm::StringSwitch<Optional<PlatformKind>>(Platform)
           .Case("ios", PlatformKind::iOS)
-          .Case("macosx", PlatformKind::OSX)
+          .Case("macos", PlatformKind::OSX)
           .Case("tvos", PlatformKind::tvOS)
           .Case("watchos", PlatformKind::watchOS)
           .Case("ios_app_extension", PlatformKind::iOSApplicationExtension)
-          .Case("macosx_app_extension",
+          .Case("macos_app_extension",
                 PlatformKind::OSXApplicationExtension)
           .Case("tvos_app_extension",
                 PlatformKind::tvOSApplicationExtension)