[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 approved this pull request.

LGTM; when we add `-fbounds-safety` in the future, we may want the language option to become an enumeration so we can distinguish between the experimental, non-experimental, and disabled bounds safety options, but we can cross that bridge when we get to it.

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