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

[Jakub Kuderski via llvm-commits]

kuhar 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).

In this specific scenario we would be replacing the same implementation with the same API. Because this is a detail, I don't think it's worth arguing over and I approved the PR as-is.

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