runtime: add a runtime function _swift_checkClassAndWarnForKeyedArchiving
runtime: add a runtime function _swift_checkClassAndWarnForKeyedArchiving

This function checks if a mangled class name is going to be written into an NSArchive.
If yes, a warning should be printed and the return value should indicate that.

TODO: print the actual warning

rdar://problem/32414508

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation