Teach the diagnostics engine about the Attr type to make reporting on semantic attributes easier (and not require hard-coded strings). This requires a getSpelling() function on the Attr class, which is table-driven. Updates a handful of cases where a hard-coded string was being used to test the functionality out. Updating associated test cases for the improved quoting.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198055 91177308-0d34-0410-b5e6-96231b3b80d8