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

[Dan Liew via cfe-commits]

delcypher wrote:

@AaronBallman Thanks for approving. I've rebased this patch and tweaked the commit message to give the context for this change. Landing now.

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