[analyzer] Correct the stale comment.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177788 91177308-0d34-0410-b5e6-96231b3b80d8
[analyzer] Warn when a nil key or value are passed to NSMutableDictionary and ensure it works with subscripting.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177789 91177308-0d34-0410-b5e6-96231b3b80d8