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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172087 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed