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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

The separations we've been making so far in `Sema` have been at a higher level of granularity than this proposal. Vlad was calling it "language extensions" but perhaps a different way to phrase it would be "unique language dialects". e.g., Objective-C is its own language, but it's technically an extension of C. Similar for things like OpenMP, etc. I don't think bounds safety is the same kind of extension in that regard; it's a handful of features allowing for extra diagnostic checking more than it's a unique language. For example, thread safety analysis can be described in exactly the same way, so should it be split off too? If we keep doing that, will splitting semantic object by language feature scale? What's the criteria for when a language feature should or should not be split? I think we can side step all of this for right now.

That said, I think grouping the bounds safety semantic bits together in a single source file is a totally reasonable thing to do (Vlad's suggestion of `SemaBoundsSafety.cpp` was along the exact lines I was thinking this should go).

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