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

[Jakub Kuderski via llvm-commits]

kuhar wrote:

> > Have you tried grepping the codebase and checking if there are some existing helpers that can be replaced by this?
> 
> I can't say I performed an exhaustive search, but apparently there is only one helper somewhere deep in MLIR:
> 
> https://github.com/llvm/llvm-project/blob/0f8615f4dc568f4d7cbf73580eef3e78f64f3bd0/mlir/lib/Dialect/SparseTensor/IR/Detail/TemplateExtras.h#L41-L45

Thanks for checking, @Endilll. Would be nice to replace this with the function from this PR.

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