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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> Out of curiosity, how does the current division of Sema fit into the "unique language dialect" classification? I've noticed there are a bunch of architecture specific Sema classes (e.g. SemaRISCV, SemaX86) and those don't really fit the classification.

They are a different group of `Sema` parts, dedicated for backend-specific code. `SemaX86` and `SemaARM` serve as a good example of what a populated backend-specific part looks like.

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