Add -Wobjc-string-compare under -Wobjc-literal-compare.

Add -Wobjc-string-compare under -Wobjc-literal-compare.

Suggested by Ted, since string literal comparison is at least slightly more
sensible than comparison of runtime literals. (Ambiguous language on
developer.apple.com implies that strings are guaranteed to be uniqued within
a translation unit and possibly across a linked binary.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@160378 91177308-0d34-0410-b5e6-96231b3b80d8