Error if an anonymous DiagGroup is referenced multiple times.

Not only is this inefficient for TableGen, it's annoying for maintenance
when renaming warning flags (unusual) or adding those flags to a group
(more likely).

This uses the new fix-it infrastructure for LLVM's SourceMgr/SMDiagnostic,
as well as a few changes to TableGen to track more source information.

llvm-svn: 172087