Fix grammatical error in diagnostic.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216221 91177308-0d34-0410-b5e6-96231b3b80d8
Simplify this logic now that -W can't be applied to remarks. No functionality change.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216222 91177308-0d34-0410-b5e6-96231b3b80d8