Removing a custom error diagnostic and replacing it with a stock one. Added a...

Removing a custom error diagnostic and replacing it with a stock one. Added a test case to ensure the diagnostic was firing properly.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195188 91177308-0d34-0410-b5e6-96231b3b80d8