`<locale>`: Facets have wrong type for named locales

[muellerj2]

[locale.facet]/5 suggests that a locale constructed with constructor locale(const char*) should provide facets of type [facet]_byname, but MSVC STL constructs facets of type [facet] instead.

Test case

#include <iostream>
#include <locale>

using namespace std;

int main() {
    locale loc("en_US");
    const auto& coll = use_facet<collate<char>>(loc);
    cout << typeid(coll).name();
}

https://godbolt.org/z/a556r5czc

Expected behavior

This should print class std::collate_byname<char>, but actually prints class std::collate<char>.

Additional context

This bug means that we can't implement the resolution of LWG-4186 (#5291) as-is.

[locale.category]/3 seems to suggest that [facet] and not [facet]_byname is the correct type for the C locale returned by locale::classic(). But this creates some tension in the spec: [locale.facet]/5 suggests that locale(const char*) should construct [facet]_byname facets, while [locale.statics]/5 says that locale::classic() should return a locale equivalent to locale("C"). Are the locales still considered equivalent if the facet types aren't the same?

Note that libc++ solves this by producing C locales with different facet types for locale("C") and locale::classic(): https://godbolt.org/z/nzW4qxja1