[clang] [BoundsSafety] Add `-fexperimental-bounds-safety` CC1 and language option and use it to tweak `counted_by`'s semantics (PR #92623)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

I'd like to understand why we need a flag for this rather than changing the behavior of `-fbounds-safety`. (I'd like to avoid a proliferation of flags unless this flag is going to gate more changes in the near future.)

https://github.com/llvm/llvm-project/pull/92623