[libc++] Create a custom locale for testing purposes

[ldionne]

Instead of using various system-provided locales to do our localization tests, we could generate a locale of our own that we control and test using that. That would greatly simplify our localization tests and would make them more robust and portable.

If we do that, we could also re-introduce some removed coverage for collated characters and equivalence classes in #159590.

[ldionne]

On macOS, I am able to create a new locale with e.g. localedef -i fr_CA.ISO8859-15.src -f maps.UTF-8 $PWD/libcxx_US.UTF-8. The fr_CA.ISO8859-15.src file was obtained from an internal locale-generation script, but I think the data provided by the CLDR project (http://cldr.unicode.org/) should be the same. The maps.UTF-8 file is a POSIX charmap generated automatically from the Unicode Character Database and Common Locale Data Repository (http://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap07.html).

[philnik777]

I think we can replace the facets of a std::locale with our own classes. That should be completely standard and portable. e.g.

#include <locale>

struct my_collate : std::collate<char> {
  using base = std::collate<char>;
  
  string_type do_transform(const char* low, const char* high) const {
    /*Do Something*/
  }
};

auto test() {
  std::locale::global(std::locale(std::locale::classic(), new my_collate));
}