[clang] [compiler-rt] [SafeStack] Introduce public interface header and deprecate builtins (PR #198292)

[Jakob Koschel via cfe-commits]

jakos-sec wrote:

> I don't think there's any reason to avoid using a header, in general, assuming it gets installed somewhere appropriate. Well, as long as you're sure the compiler doesn't need to do anything special with the function. It look like there's some special handling related to `__safestack_get_unsafe_stack_ptr` in the backend.

I've added `__safestack_get_unsafe_stack_ptr`  in this commit and it's only used in a test. There shouldn't be any other use of it in the backend, if I'm not mistaken.

> This should also come with a release note in clang/docs/ReleaseNotes.rst to alert users to the deprecation.

Also added that, thank you for the suggestion.

https://github.com/llvm/llvm-project/pull/198292