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

[Vitaly Buka via llvm-commits]

vitalybuka wrote:

@pcc @AaronBallman @efriedma-quic

Is there any rule on how to pick to introduce a builtin vs just function in complier-rt header?
SafeStack looks inconsistent with the rest, but maybe a reason for that?

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