objective-c: warn if implementation of a method in category

objective-c: warn if implementation of a method in category
masks an existing method in its primary class, class extensions,
and primary class's non-optional protocol methods; as primary
class, or one of its subclass's will implement this method.
This warning has potential of being noisy so it has its own
group.  // rdar://7020493

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@136426 91177308-0d34-0410-b5e6-96231b3b80d8