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

[Scott Linder via llvm-commits]

slinder1 wrote:

The code LGTM, but with one small nit: as it is a backport of a C++23 feature please move it to `include/llvm/ADT/STLForwardCompat.h`

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