[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:

> @AaronBallman We haven't exposed the `-fbounds-safety` flag yet. The idea was to guard it under the experimental flag called `-fexperimental-bounds-safety` as a CC1 flag for testing until we have the full feature available (the feature is work-in-progress and is going to take some time to implement). Then, we would create `-fbounds-safety` to be exposed as driver and CC1 flags. We would deprecate `-fexperimental-bounds-safety` after that point but might have to keep as an alias of `-fbounds-safety` for a release so people can switch over to `-fbounds-safety`.
> 
> Does it sound like a reasonable strategy to you? Please let us know if you have other preference.

Ah, thank you for the explanation! That sounds like a reasonable strategy to me; I was mostly worried that we'd be exposing two driver flags for this and it wasn't clear what the distinction was. But for a CC1 flag to enable testing and for early adopters, it seems reasonable. Thanks!

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