Updated the wording for a diagnostic to be more grammatically correct, and use a %select. Also ensure that nested namespace definitions are diagnosed properly. Both changes are motivated by post-commit feedback from r221580.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221581 91177308-0d34-0410-b5e6-96231b3b80d8