Fix build error with creating 'AvailabilityAttr', after r263958.
Fix build error with creating 'AvailabilityAttr', after r263958.

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang