[clang] [Clang] Add __has_target_builtin macro (PR #126324)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman requested changes to this pull request.

Marking as requesting changes so we don't accidentally land, because I've got questions about whether we want to go down this route.

1) GCC has `__has_builtin`, so how do they handle offloading targets? Do they have the same odd behavior where `__has_builtin` returns true for builtins it cannot actually emit code for?
2) Given that `__has_target_builtin` seems to have the semantics everyone would expect from `__has_builtin`, do we want to consider deprecating `__has_builtin` so that downstreams have time to adjust but we eventually end up with a less confusing builtin?

It really seems to me that `__has_builtin` has a broken design because of compilations where we try to hide a two-pass compilation as though it were one-pass and it seems like we're going to confuse folks with that behavior. If I could wave a magic wand, I would say `__has_builtin` should behave exactly how `__has_target_builtin` is behaving in this PR and that a reliance on `__has_builtin` behaving how it does today is relying on a bug.

https://github.com/llvm/llvm-project/pull/126324