<xloctime>: Fix time_put::do_put to correctly handle %c and %r formatting in "C" locale

[ferhatgec]

Fixes #6134.

Description
The C standard requires the %c and %r specifiers to be equivalent to %a %b %e %T %Y and %I:%M:%S %p respectively in the "C" locale (N3220 § 7.29.3.5/7). While the standard C strftime might handle this correctly, the internal UCRT functions _Strftime and _Wcsftime used by <xloctime> do not provide this expected output. This causes a mismatch when using std::time_put::do_put.

This PR introduces a workaround in time_put::do_put. When _Iosbase.getloc() == locale::classic(), it explicitly overrides the format strings for the %c and %r specifiers with the standard-mandated sequences before passing them to _Strftime / _Wcsftime. Since this problem only occurs in "C" locale, this workaround resolves the issue without affecting others.

Testing

• The workaround allows the failing libcxx test to pass.
• Removed std/localization/locale.categories/category.time/locale.time.put/locale.time.put.members/put2.pass.cpp from tests/libcxx/expected_results.txt.

[StephanTLavavej]

This needs to be clang-formatted with the current version of Clang shipping in VS Insiders (currently Clang 22.1.3). We recommend configuring your editor to use clang-format; if you use VSCode, our repo will enable format-on-save for C++ files. The configuration setting for the C/C++ Extension to hook up clang-format is "C_Cpp.clang_format_path": "C:/Program Files/Microsoft Visual Studio/18/Insiders/VC/Tools/Llvm/x64/bin/clang-format.exe", with the default path.

[ferhatgec]

Sorry, my bad. Didn't realize my IDE was using my own .clang-format.

[ferhatgec]

It seems the CI is failing because P0355R7_calendars_and_time_zones_formatting contains test cases that are affected by our changes.
Replacing:

assert(format(STR("{:%c, %x, %X}"), ltf) == STR("04/19/21 01:02:03, 04/19/21, 01:02:03"));

with:

assert(format(STR("{:%c, %x, %X}"), ltf) == STR("Mon Apr 19 01:02:03 2021, 04/19/21, 01:02:03"));

and replacing:

assert(format(STR("{:%c, %x, %X}"), zt) == STR("04/19/21 08:16:17, 04/19/21, 08:16:17"));

with:

assert(format(STR("{:%c, %x, %X}"), zt) == STR("Mon Apr 19 08:16:17 2021, 04/19/21, 08:16:17"));

Doing these seems to resolve the issue, at least on my PC.

[MattStephanson]

The C Standard (N3685 7.31.3.6/7) and the informative "Application Usage" part of the POSIX spec both say that
"In the C [or POSIX] locale, the E and O modifiers are ignored". The UCRT is actually doing the opposite of this PR; it unconditionally replaces %r in the C locale but only replaces non-alternative %c (I don't have a link for this but you can find it in the UCRT expand_time source).

[ferhatgec]

Thanks for the catch. I've made some changes, now in C locale, %r, %Er and %Or are treated in the same way.

[MattStephanson]

LGTM, thanks!