[analyzer] Fix null tracking for the given test case, by using the proper state and removing redundant code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178933 91177308-0d34-0410-b5e6-96231b3b80d8
[analyzer] Remove another redundancy from trackNullOrUndef

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178934 91177308-0d34-0410-b5e6-96231b3b80d8
[analyzer] Reword error messages for nil keys and values of NSMutableDictionary.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178935 91177308-0d34-0410-b5e6-96231b3b80d8