Enforce std::time_base::mdy for "C" locale

[fsb4000]

Fixes #3346

1. I found out that the tests work on libc++.
2. I looked how libc++ detect date_order. It has a some date and run std::strftime(mbstr, sizeof(mbstr), "%x", std::localtime(&t)); and analyse its output.
3. I looked at strftime implementation. It creates _LocaleUpdate object. and passes nullptr to it.
   _LocaleUpdate constructor looks like that (pseudo-code)

if (locale)
{
    _locale_pointers = *locale;
}
else if (is_it_C_locale())
{
    _locale_pointers = some_predefined_values;
}
else
{
   query runtime data to fill _locale_pointers from the current locale.
}

4. So std::strftime uses mdy for "C" locale and we can do too. I found another places where we did a special case for "C" locale.
   

   STL/stl/src/xstrcoll.cpp

   Lines 57 to 69 in 8b081e2

   	if (locale_name == nullptr) {
   	int ans = memcmp(string1, string2, n1 < n2 ? n1 : n2);
   	ret = (ans != 0 || n1 == n2 ? ans : n1 < n2 ? -1 : +1);
   	} else {
   	ret = __crtCompareStringA(locale_name, SORT_STRINGSORT, string1, n1, string2, n2, codepage);
   	if (ret == 0) {
   	errno = EINVAL;
   	ret = _NLSCMPERROR;
   	} else {
   	ret -= 2;
   	}
   	}

   
   

   STL/stl/src/_tolower.cpp

   Lines 52 to 58 in 8b081e2

   	if (locale_name == nullptr) {
   	if (c >= 'A' && c <= 'Z') {
   	c = c + ('a' - 'A');
   	}
   	return c;
   	}

   
   

   STL/stl/src/_toupper.cpp

   Lines 51 to 57 in 8b081e2

   	if (locale_name == nullptr) {
   	if (c >= 'a' && c <= 'z') {
   	c = c - ('a' - 'A');
   	}
   	return c;
   	}

[StephanTLavavej]

I believe this behavioral change is desirable - we'll properly detect when we should be using the "C" locale, instead of picking up LOCALE_NAME_USER_DEFAULT for the date order - but as it affects legacy code in my magical weak point, I'd like a second maintainer review here.

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for investigating and fixing this longstanding bug that was affecting several contributors! 🌎 🌍 🌏