Put some diagnostics in DiagnosticCommonKinds.td in a category, mirroring what they are in other .td files.

I really dislike the copy-pasting of the category strings.  If there is a better
way to do this with TableGen, please advise.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181494 91177308-0d34-0410-b5e6-96231b3b80d8