`<xloctime>`: `time_get::do_get` uses the wrong format for `%c` and `%x`

[cpplearner]

Describe the bug

MSVC STL's time_get::do_get hardcodes %c as equivalent to %b %d %H : %M : %S %Y, and %x as equivalent to %d / %m / %y.

STL/stl/inc/xloctime

Lines 455 to 457 in 117ca96

	case 'c':
	_First = _Getfmt(_First, _Last, _Iosbase, _State, _Pt, "%b %d %H : %M : %S %Y");
	break;

STL/stl/inc/xloctime

Lines 549 to 551 in 117ca96

	case 'x':
	_First = _Getfmt(_First, _Last, _Iosbase, _State, _Pt, "%d / %m / %y");
	break;

This doesn't match what strftime produces:

• In the "C" locale, %c is equivalent to %a %b %e %T %Y, and %x is equivalent to %m/%d/%y. MSVC notably doesn't parse %a for the former, and has the wrong order for the latter.
• Other locales might have very different formats.

Fixing this would be a behavior change (even if ABI-preserving). I don't know if it's worth the trouble.

This affects libcxx test std/localization/locale.categories/category.time/locale.time.get/locale.time.get.members/get_one.pass.cpp.

Command-line test case

D:\test>type test-time-get-do-get.cpp

#include <cassert>
#include <ctime>
#include <iostream>
#include <locale>

using I = const char*;

class my_facet : public std::time_get<char, I>
{
public:
    explicit my_facet(std::size_t refs = 0)
        : time_get(refs) {}
};

int main() {
    const my_facet f(1);
    std::ios str(nullptr);
    std::ios_base::iostate err;
    std::tm t;

    {
        const char in[] = "Thu Jun  6 09:49:10 2009";
        err = std::ios_base::goodbit;
        t = std::tm();
        I i = f.get(I(in), I(in+sizeof(in)-1), str, err, &t, 'c');
        assert(i == in+sizeof(in)-1);
    }
}

D:\test>cl /EHsc test-time-get-do-get.cpp

Microsoft (R) C/C++ Optimizing Compiler Version 19.50.35725 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

test-time-get-do-get.cpp
Microsoft (R) Incremental Linker Version 14.50.35725.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:test-time-get-do-get.exe
test-time-get-do-get.obj

D:\test>test-time-get-do-get.exe

Assertion failed: i == in+sizeof(in)-1, file test-time-get-do-get.cpp, line 26

Expected behavior

Assertion passed.

STL version

117ca96

Additional context