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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> > That's what the initial proposal / patch was, but that got reverted because it broke some CUDA on Arm sources. See #121839.
> 
> Yeah, but I'm suggesting that seems like a bug with CUDA on Arm sources. (But it also depends on how GCC behaves with offload targets; if they have the same wacky behavior, then it's not a bug.) If we agree that's a bug, it may change how we want to approach the fix here.

If changing the sources in an option I'm in favor of making `__has_builtin` work as the user would expect, but hacking around this stuff is par for the course in offloading land.

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