Give warning when [super method] is not called

Recent versions of llvm have added an attribute that indicates that subclasses must call super:

@interface Barn:NSObject
- (void)openDoor NS_REQUIRES_SUPER;
@end

@implementation Barn
- (void) openDoor
{
    ;
}
@end

@interface HorseBarn:Barn
@end
@implementation HorseBarn
- (void) openDoor
{
    ;
}
@end

Compiling the above produces the warning:

Method possibly missing a [super openDoor] call