<locale>: missing some locale names

Describe the bug
Some libc++ tests use UTF_8 locale. But they use ANSI locale on Windows. Also the tests may fail for another reasons.

Command-line test case

#ifndef PLATFORM_SUPPORT_H
#define PLATFORM_SUPPORT_H

// locale names
#ifdef _WIN32
    // WARNING: Windows does not support UTF-8 codepages.
    // Locales are "converted" using https://docs.moodle.org/dev/Table_of_locales
#   define LOCALE_en_US           "en-US"
#   define LOCALE_en_US_UTF_8     "en-US"
#   define LOCALE_cs_CZ_ISO8859_2 "cs-CZ"
#   define LOCALE_fr_FR_UTF_8     "fr-FR"
#   define LOCALE_fr_CA_ISO8859_1 "fr-CA"
#   define LOCALE_ru_RU_UTF_8     "ru-RU"
#   define LOCALE_zh_CN_UTF_8     "zh-CN"
#elif defined(__CloudABI__)
    // Timezones are integrated into locales through LC_TIMEZONE_MASK on
    // CloudABI. LC_ALL_MASK can only be used if a timezone has also been
    // provided. UTC should be all right.
#   define LOCALE_en_US           "en_US"
#   define LOCALE_en_US_UTF_8     "en_US.UTF-8@UTC"
#   define LOCALE_fr_FR_UTF_8     "fr_FR.UTF-8@UTC"
#   define LOCALE_fr_CA_ISO8859_1 "fr_CA.ISO-8859-1@UTC"
#   define LOCALE_cs_CZ_ISO8859_2 "cs_CZ.ISO-8859-2@UTC"
#   define LOCALE_ru_RU_UTF_8     "ru_RU.UTF-8@UTC"
#   define LOCALE_zh_CN_UTF_8     "zh_CN.UTF-8@UTC"
#else
#   define LOCALE_en_US           "en_US"
#   define LOCALE_en_US_UTF_8     "en_US.UTF-8"
#   define LOCALE_fr_FR_UTF_8     "fr_FR.UTF-8"
#   ifdef __linux__
#       define LOCALE_fr_CA_ISO8859_1 "fr_CA.ISO-8859-1"
#       define LOCALE_cs_CZ_ISO8859_2 "cs_CZ.ISO-8859-2"
#   else
#       define LOCALE_fr_CA_ISO8859_1 "fr_CA.ISO8859-1"
#       define LOCALE_cs_CZ_ISO8859_2 "cs_CZ.ISO8859-2"
#   endif
#   define LOCALE_ru_RU_UTF_8     "ru_RU.UTF-8"
#   define LOCALE_zh_CN_UTF_8     "zh_CN.UTF-8"
#endif

#include <stdio.h>
#include <stdlib.h>
#include <codecvt>
#include <locale>
#include <string>
#if defined(_WIN32) || defined(__MINGW32__)
#   include <io.h> // _mktemp_s
#else
#   include <unistd.h> // close
#endif

#if defined(_NEWLIB_VERSION) && defined(__STRICT_ANSI__)
// Newlib provides this, but in the header it's under __STRICT_ANSI__
extern "C" {
  int mkstemp(char*);
}
#endif

#ifndef __CloudABI__
inline
std::string get_temp_file_name()
{
#if defined(__MINGW32__)
    char Path[MAX_PATH + 1];
    char FN[MAX_PATH + 1];
    do { } while (0 == GetTempPath(MAX_PATH+1, Path));
    do { } while (0 == GetTempFileName(Path, "libcxx", 0, FN));
    return FN;
#elif defined(_WIN32)
    char Name[] = "libcxx.XXXXXX";
    if (_mktemp_s(Name, sizeof(Name)) != 0) abort();
    return Name;
#else
    std::string Name;
    int FD = -1;
    do {
        Name = "libcxx.XXXXXX";
        FD = mkstemp(&Name[0]);
        if (FD == -1 && errno == EINVAL) {
            perror("mkstemp");
            abort();
        }
    } while (FD == -1);
    close(FD);
    return Name;
#endif
}

#ifdef _LIBCPP_HAS_OPEN_WITH_WCHAR
inline
std::wstring get_wide_temp_file_name()
{
    return std::wstring_convert<std::codecvt_utf8_utf16<wchar_t> >().from_bytes(
        get_temp_file_name());
}
#endif // _LIBCPP_HAS_OPEN_WITH_WCHAR

#endif // __CloudABI__

#endif // PLATFORM_SUPPORT_H

Expected behavior
Tests should pass

STL version
Microsoft Visual Studio Community 2019 Preview Version 16.8.0 Preview 2.0

Additional context
Skipped libc++ tests
https://github.com/microsoft/STL/blob/06827feb4cdc4d2328dfbfab9fd5302de6058dd9/tests/libcxx/expected_results.txt#L599-L629

Start with the skipped libc++ entries in tests/libcxx/expected_results.txt at the linked lines and compare them with the locale definitions in the reported platform-support code. Run the affected tests with the listed Visual Studio configuration, then verify that the locale cases no longer need to be skipped and pass on Windows.