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

[Yeoul Na via cfe-commits]

rapidsna 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 are added. I can make it a CC1 only flag for now. Would it work for you better?

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