[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:

This is based on #70480 but removes the driver part of the change and makes the flag a CC1 flag only.

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