allow %x for formatting year_month_day

[fsb4000]

Fixes #2761

I'm not sure if it's correct.

C++ standard is unclear: https://eel.is/c++draft/time.format#6

But cppreference says that %x is supported for year_month_day, year_month_day_last, year_month_weekday and year_month_weekday_last

[StephanTLavavej]

I believe that this is correct (although the Standard is unusually elliptical in this area, these types do provide the necessary information).

[barcharcraz]

This looks good. Ideally, we would test the locale specific orderings, but with current test coverage this looks like a strict improvement, even if the locale stuff were to be broken so I don't feel it needs those tests to be merged.

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for fixing this chronat bug! 📅 ✏️ 🎉