[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.

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