<xloctime>: Fix time_get::do_get format strings for %c and %x

[Brandon Pacewic (BrandonPacewic)]

Fixes #6129.

time_get::do_get hardcoded the wrong format strings for %c and %x in the C locale:

• %c was "%b %d %H : %M : %S %Y", missing the leading weekday field entirely. It should be "%a %b %e %T %Y" to match what strftime("%c") produces.
• %x was "%d / %m / %y" (day/month order), but should be "%m/%d/%y" (month/day order) to match strftime("%x").

As the original open issue describes, this is an ABI preserving behavior change. The existing test Dev11_0836436_get_time is updated accordingly and its %x test inputs were written for the old day first format and its comment cited "%d / %m / %y" directly.

The libcxx test std/localization/locale.categories/category.time/locale.time.get/locale.time.get.members/get_one.pass.cpp still fails due to a separate pre-existing bug where do_get does not handle the %% specifier (falls to default: failbit) (#6130). Its entry in expected_results.txt is left in place.

Also updates P0355R7_calendars_and_time_zones_io test inputs for %c and %x, since std::chrono::parse routes those format specifiers through time_get::do_get and the existing inputs were written against the old incorrect format strings.

Force push note 🐱

I force pushed during early preparation of this PR to fix a commit message and add a missing `test.lst` entry before any review had begun. I understand that force pushing during active review is disruptive and I will not do so going forward. 🙃

[Stephan T. Lavavej (StephanTLavavej)]

Thanks! 😻 I pushed a test change as recommended by Matt Stephanson (@MattStephanson).

[S. B. Tam (cpplearner)]

This fixes time_get so that it can parse the output of strftime in the "C" locale, which is an improvement.

The format still doesn't match strftime when the locale is not "C", but I think we shouldn't support that scenario. I filed LWG-4541 in the hope that we can get an explicit permission to do whatever we want with locale-dependent specifiers like %c or %x.

[Stephan T. Lavavej (StephanTLavavej)]

I'm mirroring this to the MSVC-internal repo. Please notify me if any further changes are pushed, otherwise no action is required.

[Stephan T. Lavavej (StephanTLavavej)]

Thanks for fixing this bug! ⏱️ 🗺️ 🌏