New ObjC warning: circular containers.

This commit adds new warning to prevent user from creating 'circular containers'.

Mutable collections from NSFoundation allows user to add collection to itself, e.g.: 

NSMutableArray *a = [NSMutableArray new]; 
[a addObject:a]; 

The code above leads to really weird behaviour (crashes, 'endless' recursion) and 
retain cycles (collection retains itself) if ARC enabled.

Patch checks the following collections: 
  - NSMutableArray, 
  - NSMutableDictionary, 
  - NSMutableSet, 
  - NSMutableOrderedSet, 
  - NSCountedSet. 

llvm-svn: 231265