Don't include <xmemory> in <optional> and <variant>

[frederick-vs-ja]

Separated from #3623.

std::variant and std::optional forbid using an array type as component, so it's unnecessary to include <xmemory> to obtain _Destroy_in_place.

I think it's OK to write <xutility> for short. Several other headers, including conditionally included <compare>, are already handled in <xutility>. Edit: dropping this due to review comments.

Edit: I also need to reduce the numeric_limits depedency (needed for EDG?). I expect that the current approach is transient.

[strega-nil-ms]

Seems reasonable, but I don't like removing explicit dependencies, personally (since the throughput gains are negligible).

[StephanTLavavej]

No change requested: I observe that we're storing remove_cv_t<_Ty> _Value; and saying only ~_Ty(). This seems to be acceptable according to the Standardese, and all compilers accept, so I don't think we need to add remove_cv_t here.

[StephanTLavavej]

Looks good to me, thanks! I think that the breaking change impact here is limited - <optional> and <variant> are C++17 (so legacy C++14 code is unaffected), and the difference between <xmemory> and just <xutility> isn't massive. <limits> as you found and <tuple> in C++20 mode seem notable but not highly problematic.

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for finding and implementing this throughput improvement! 🚀 🐇 🐆