CoreFoundation: guard some declarations
CoreFoundation: guard some declarations

These declarations are only implemented for the macOS target, so do not
provide a declaration on other targets.

parkera/swift-corelibs-foundation,e78l/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,e78l/swift-corelibs-foundation,parkera/swift-corelibs-foundation,e78l/swift-corelibs-foundation,parkera/swift-corelibs-foundation,apple/swift-corelibs-foundation,parkera/swift-corelibs-foundation,parkera/swift-corelibs-foundation,e78l/swift-corelibs-foundation,e78l/swift-corelibs-foundation,apple/swift-corelibs-foundation