[clang] [Clang] Add GCC's __builtin_stack_address() to Clang. (PR #121332)

[John McCall via cfe-commits]

rjmccall wrote:

I agree with Nikita. I have no problem with supporting this builtin, but it feels like something that should be expressed portably as an IR intrinsic and then get lowered in the backend rather than needing frontend work for every target.

https://github.com/llvm/llvm-project/pull/121332