[clang] [RFC] Initial implementation of P2719 (PR #113510)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

I am comfortable landing the changes ahead of acceptance at WG21 plenary so long as it's understood that if WG21 makes changes which break us, we'll implement those changes. And based on that, I think setting the feature test macro is fine.

https://github.com/llvm/llvm-project/pull/113510