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

[Vlad Serebrennikov via llvm-commits]

Endilll wrote:

> I'd land it together, otherwise this change has no uses in the codebase. And the change is very simple.

This is driven by my work on Clang enums, so I intentionally factored this PR out so that this useful facility is not tied to any particular usage (that might get reverted). I'm going to commit several NFC patches tomorrow that use this.

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