Consolidate several attribute argument diagnostics into a single, selectable diagnostic.  This makes the diagnostic more consistent.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186940 91177308-0d34-0410-b5e6-96231b3b80d8
Removed a redundant diagnostic and replaced it with a more standard one.  Added a test case for the diagnostic.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186942 91177308-0d34-0410-b5e6-96231b3b80d8