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

[Nick Sarnie via cfe-commits]

sarnex wrote:

@AaronBallman What do you recommend for next steps here? Should we wait until GCC makes a decision in [this issue](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118882)?

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