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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> > Are there any blockers for this left?
> 
> I don't believe so, the consensus on the thread seemed to be to change `__has_builtin` behavior rather than go with this PR though, right?

Yeah, this PR introduces the `__has_target_builtin` so I was wondering if there was a blocker on updating it. If @sarnex is too busy I'll probably do it.

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