Implement P2445R1 forward_like()

[frederick-vs-ja]

Fixes #2931.

Intentionally don't call std::move, std::forward, or std::as_const. Also move _Can_reference to <utility> for error message.

[CaseyCarter]

LGTM!

[strega-nil-ms]

I don't really have a "problem" with how it's written right now, but I would prefer it if this was written closer to the wording of the paper; something like:

using _UnrefT = remove_reference_t<_Ty>;
using _UnrefU = remove_reference_t<_Uty>;
if constexpr (is_const_v<_UnrefT>) {
    // _Copy_const = const _UnrefU
    if constexpr (is_rvalue_reference_v<_Ty&&>) {
        return static_cast<const _UnrefU&&>(_Ux);
    } else {
        return static_cast<const _UnrefU&>(_Ux);
    }
} else {
    // _Copy_const = _UnrefU
    if constexpr (is_rvalue_reference_v<_Ty&&>) {
        return static_cast<_UnrefU&&>(_Ux);
    } else {
        return static_cast<_UnrefU&>(_Ux);
    }
}

[StephanTLavavej]

@frederick-vs-ja @CaseyCarter @strega-nil-ms Thanks, this looks good! I added a preprocessor comment and slightly expanded the test coverage (mostly out of principle, not any real concern about library or compiler bugs).

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for implementing this feature - I like to see the STL moving forward! 😹 😻 🚀