[clang] [plugins] Let plugins put diagnostics in user-controllable warning groups (PR #208538)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> @AaronBallman, @aengelke, do we have a chance to get this in before branching -- it's important for downstream plugins...

I'm very uncomfortable designing what seems to be a non-trivial piece of plugin functionality in the 5 days left before branching.

https://github.com/llvm/llvm-project/pull/208538