objc_subclassing_restricted attribute bug

Hi! Found a bug. If you add objc_subclassing_restricted attribute to both parent and child classes it compiles without any error. So attribute appears to be useless, as you can’t rely on it.

Code to reproduce bug:

#import <Foundation/Foundation.h>

#define FINAL attribute((objc_subclassing_restricted))

FINAL
@interface Parent : NSObject
@end
@implementation Parent
@end

FINAL
@interface Child : Parent
@end
@implementation Child
@end

int main(int argc, const char * argv[]) {
return 0;
}

Hi! Found a bug. If you add objc_subclassing_restricted attribute to both
parent and child classes it compiles without any error. So attribute appears
to be useless, as you can't rely on it.

Do you have -Wunknown-attributes enabled? You should get a diagnostic
telling you that the unknown attribute is unsupported, as that is not
an attribute Clang understands (it's not listed in Attr.td).
(http://coliru.stacked-crooked.com/a/eb3ba9326e8648ea)

~Aaron