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

[Aaron Ballman via llvm-commits]

https://github.com/AaronBallman approved this pull request.

The changes look reasonable to me, and I presume you intend to use this in-tree for the enum changes you've been working on in Clang. But I added a few more reviewers just to get more sets of eyes on the changes; please don't land until at least one more reviewer has accepted.

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