[clang] [clang][NFC] Move Bounds Safety Sema code to `SemaBoundsSafety.cpp` (PR #99330)

[Kees Cook via cfe-commits]

kees wrote:

> @rapidsna @hnrklssn @bwendling @kees Please let me know if you have any concerns about this refactor.

Yeah, FWIW, this is fine by me. I just want to make sure that we don't hide stuff behind `-fbounds-safety` that doesn't need to be behind a flag (e.g. `counted_by`, `sized_by` and their application to struct members, function prototypes, etc).


https://github.com/llvm/llvm-project/pull/99330