AST: Fix AvailabilityContext's containment check for platforms.

The unavailable platform kind of the outer AvailabilityContext must not inherit
availability from the platform of the inner context.

Author: tshortli

lib/AST/AvailabilityContext.cpp

@@ -91,8 +91,8 @@ bool AvailabilityContext::PlatformInfo::isContainedIn(
   if (IsUnavailable && other.IsUnavailable) {
     if (UnavailablePlatform != other.UnavailablePlatform &&
         UnavailablePlatform != PlatformKind::none &&
-        !inheritsAvailabilityFromPlatform(UnavailablePlatform,
-                                          other.UnavailablePlatform))
+        inheritsAvailabilityFromPlatform(UnavailablePlatform,
+                                         other.UnavailablePlatform))
       return false;
   }
 

test/attr/attr_availability_transitive_osx_appext.swift

@@ -323,17 +323,16 @@ extension ExtendMe {
     osx_extension() // expected-error {{'osx_extension()' is unavailable in application extensions for macOS}}
   }
 
-  // FIXME: Reenable
-//  @available(OSX, unavailable)
-//  func osx_app_extension_extension_osx_method(
-//    _: NeverAvailable,
-//    _: OSXUnavailable,
-//    _: OSXAppExtensionsUnavailable
-//  ) {
-//    never()
-//    osx()
-//    osx_extension()
-//  }
+  @available(OSX, unavailable)
+  func osx_app_extension_extension_osx_method(
+    _: NeverAvailable,
+    _: OSXUnavailable,
+    _: OSXAppExtensionsUnavailable
+  ) {
+    never() // expected-error {{'never()' is unavailable}}
+    osx()
+    osx_extension()
+  }
 
   @available(OSXApplicationExtension, unavailable)
   func osx_app_extension_extension_osx_app_extension_method(