[clang] Give the __COUNTER__ pedantic diagnostic a group (PR #218669)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> The current plan is to try to get this diagnostic landed ASAP and backport it to the 23.x branch. It's possible we're going to miss the window though because today is supposed to be the release according to schedule. But that's why there's no release note for the change currently. If I miss the window, I'll add a release note.

I just heard back from the release managers that we missed the window for 23.1.0, but can file a backport request for a future point release, so I'll add the release note here in case no further point releases are made.

https://github.com/llvm/llvm-project/pull/218669