[ClangImporter] Update diagnostic to refer to "macOS" instead of "OS X"

Author: devincoughlin

lib/ClangImporter/ClangImporter.cpp

@@ -1703,7 +1703,7 @@ PlatformAvailability::PlatformAvailability(LangOptions &langOpts)
   case PlatformKind::OSX:
   case PlatformKind::OSXApplicationExtension:
     deprecatedAsUnavailableMessage =
-        "APIs deprecated as of OS X 10.9 and earlier are unavailable in Swift";
+        "APIs deprecated as of macOS 10.9 and earlier are unavailable in Swift";
     break;
 
   default:

test/ClangImporter/availability_macosx.swift

@@ -6,7 +6,7 @@ import Foundation
 import AvailabilityExtras
 
 func test_unavailable_because_deprecated() {
-  _ = NSRealMemoryAvailable() // expected-error {{APIs deprecated as of OS X 10.9 and earlier are unavailable in Swift}}
+  _ = NSRealMemoryAvailable() // expected-error {{APIs deprecated as of macOS 10.9 and earlier are unavailable in Swift}}
 }
 
 func test_swift_unavailable_wins() {

test/SourceKit/DocSupport/doc_clang_module.swift.response

@@ -7014,7 +7014,7 @@ var FooSubUnnamedEnumeratorA1: Int { get }
             key.kind: source.lang.swift.attribute.availability,
             key.platform: source.availability.platform.osx,
             key.is_unavailable: 1,
-            key.message: "APIs deprecated as of OS X 10.9 and earlier are unavailable in Swift",
+            key.message: "APIs deprecated as of macOS 10.9 and earlier are unavailable in Swift",
             key.deprecated: "10.1"
           }
         ],