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

[Henrik G. Olsson via cfe-commits]

hnrklssn wrote:

> 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 don't have a strong opinion on whether this should be split up, but I just wanted to point out that `-fbounds-safety` is also a language extension, so it does kind of fit your description of other things separated out. It's currently in the process of being upstreamed which is why it's small, but it'd be easier to split now than wait until it's reached a certain size, if we do want to split it eventually.

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