Replace tag dispatch with if constexpr for _Choose_pocca, _Choose_pocma

[AlexGuteniev]

Towards #189

[StephanTLavavej]

Thanks! I've validated and will push changes.

[StephanTLavavej]

FYI @CaseyCarter, I pushed changes after you approved. As @AlexGuteniev noted, in #2217 (comment) you had asked for <xhash> to be left as-is to avoid unnecessary refactoring (which is greatly appreciated 😻), but after going on vacation and looking at this with a fresh eye, I noticed that the if constexpr modernization is introducing code duplication when before we had centralization with the tag dispatch overloads. I think that for maintainability, this justifies slightly larger changes to preserve code centralization (and follow the pattern in other containers).

The necessary code is a simple guard type, expressed with a const _Hash& since we never need to dismiss it. As I mentioned in #2217 (comment) , the behavioral changes are limited to the STL-internal checks in the test suite, and result in invariants being checked in the event of exceptions (which is totally beneficial).

(We could actually drop the guard type and repeat the invariant checks before every return - which would be less severe than duplicating the actual logic - but I felt that the guards were the simplest.)

Requesting double-final review as these changes are a bit larger than the typical stuff I push.

[StephanTLavavej]

I'm mirroring this to an MSVC-internal PR; please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for improving this code - all shall love if constexpr and despair! 😻 🚀 🎉