[llvm] [ADT] Backport std::to_underlying from C++23 (PR #70681)

[Aaron Ballman via llvm-commits]

AaronBallman wrote:

> > @kuhar Would it be fine to leave such replacement out of scope of this PR?
> 
> I'd land it together, otherwise this change has no uses in the codebase. And the change is very simple.

FWIW, it's pretty typical for follow-up commits to start using the new ADT functionality; this separation makes it a bit easier to revert if necessary (when the ADT is fine in general but its specific use as a replacement is problematic for some reason).

https://github.com/llvm/llvm-project/pull/70681