Sema: Emit a warning for non-null terminated format strings and other pathological cases.

PR18905.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201795 91177308-0d34-0410-b5e6-96231b3b80d8
Remove unused diagnostic.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201796 91177308-0d34-0410-b5e6-96231b3b80d8