[clang] [Driver][BoundsSafety] Add -fexperimental-bounds-safety flag (PR #70480)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> > In addition, I think our convention is to add the option when the feature is ready, rather than add a no-op option, than build functional patches on top of it.
> 
> @MaskRay This feature will involve a lot of incremental patches. And we will still need an experimental flag to test the incremental functionalities that we will be adding. I can make it a CC1 only flag for now. Would it work for you better?

Yes. This sound good to me.

You will need to remove the lib/Driver change and the language compatibility check, then.

I think the patch should still be held off until the first patch that does some basic functionality is ready.

https://github.com/llvm/llvm-project/pull/70480