Unless -enable-resilient-objc-class-stubs is passed in, these cases
are not supported, so now we diagnose them instead of asserting or
failing to link.
Note the behavior change here; classes with resilient ancestry were
previously isObjC(). However this is wrong since isObjC() means
"statically visible to Objective-C via the generated header".
After this patch, isObjC() only returns true for a class with resilient
ancestry if -enable-resilient-objc-class-stubs is passed in.
