Merge pull request swiftlang#24950 from mikeash/objc-getclass-disable-conditionally

[Test] Add availability to the GenericMangled test in objc_getClass.swift

Author: mikeash

test/Interpreter/SDK/objc_getClass.swift

@@ -179,6 +179,7 @@ testSuite.test("GenericMangled")
                 reason: "objc_getClass hook not present"))
   .requireOwnProcess()
   .code {
+  guard #available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *) else { return }
   requireClass(named:   "_TtC4main24ConstrainedSwiftSubclass",
                demangledName: "main.ConstrainedSwiftSubclass")
   requireClass(named:   "_TtC4main26ConstrainedSwiftSuperclass",