Teach the diagnostics engine about the Attr type to make reporting on semantic...

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.

llvm-svn: 198055