<utility>: Implement to_underlying()

[Daniel Marshall (SuperWig)]

As it seems remaining C++20 is nearing completion; I figure now's a good time to get started on C++23, starting with this incredibly small feature.

Fixes #1681.

[Stephan T. Lavavej (StephanTLavavej)]

Like #1478, this is blocked by the absence of _HAS_CXX23 in vcruntime. We need to add it there (or yvals_core.h as a workaround), now that /std:c++20 and /std:c++latest are distinguishable options.

[Daniel Marshall (SuperWig)]

It's things like this that'll stop us from completing C++23 by next year.

[Casey Carter (CaseyCarter)]

It's things like this that'll stop us from completing C++23 by next year.

Next year? I thought we were freezing the C++23 ABI in a month or two. 👿

[Stephan T. Lavavej (StephanTLavavej)]

This PR needs to merge main and resolve a conflict in yvals_core.h.

[Daniel Marshall (SuperWig)]

Uh, why didn't that skip CI?

[Stephan T. Lavavej (StephanTLavavej)]

Uh, why didn't that skip CI?

Apparently, if that tag appears in a merged commit, it will skip a CI build for main - however, it will not skip a PR build. 😿

[Stephan T. Lavavej (StephanTLavavej)]

Thanks for implementing this C++23 feature - it may be small, but I think it'll be the favorite of many users. 😻 🎉