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

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar commented:

Makes sense, I saw this implemented in half a dozen of downstream llvm-based projects.

Have you tried grepping the codebase and checking if there are some existing helpers that can be replaced by this?

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