[clang] [Bounds Safety][NFC] Add `SemaBoundsSafety` class and move existing Sema checks there (PR #98954)

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll requested changes to this pull request.

Unfortunately, I think this should be held off until we have a bigger picture for the future of `Sema`. I'll elaborate below.

If you take a close look at the existing set of `Sema` parts, it's almost entirely comprised of other languages or language extensions (from C and C++ standpoint) and backends. They were considered natural enough to be split off `Sema`, which, among other things, means that it's easy to explain their place in the big picture, like I did in the previous sentence.

I drove the effort to split `Sema`, and it has stalled, because it's not clear what to do with what's left there. We had several offline discussions, but we didn't find a solution that would substantially improve status quo.

I don't think this patch makes it easier to explain people where things are in this codebase, because (I think) `counted_by` is a declaration attribute, and we already have place for them. Having only one function also makes it a very small part of `Sema`, which doesn't sound compelling. A bigger picture this fits in would be a compelling argument (like it was for small backend parts), but to my knowledge there's none. Coming up with one and getting maintainers on board is certainly out of scope of this PR, so don't feel obligated to do that. Hopefully I'll get back to this whole topic later.

CC @AaronBallman 

https://github.com/llvm/llvm-project/pull/98954