Before the change from @_objcImplementation to @objc @implementation, if a member was unrepresentable in ObjC, it would become implicitly `final`. After that change, this is now an error. We do want a diagnostic here, but we don’t want to break backwards compatibility for early adopters.
Soften the error to a warning when the old @objcImplementation syntax is used.
Fixes rdar://129247349.
…for extensions. This change also removes @implementation(CategoryName); you should attach the category name to the @objc attribute instead. And there are small changes to how much checking the compiler will do on an @objc @implementation after the decl checker has discovered a problem with it.