[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]

AaronBallman wrote:

> To your point about proliferation of flags. `-fexperimental-bounds-safety` **is the CC1 flag** that guard the `-fbounds-safety` feature in our internal Clang (modulo the name, we don't have the `experimental-` prefix internally). It will **need to exist** as we continue upstream more of our implementation that depends on this flag.

Okay, let me make sure I'm on the same page -- what we've been calling `-fbounds-safety` but never actually exposed as a CC1 or driver option is actually going to be exposed under the name `-fexperimental-bounds-safety` as a CC1 option? So there's not going to be two flags for users to have to distinguish between?

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