Port QLocale's locale name parsing to QAnyStringView

The functions surrounding locale names (qt_splitLocaleName, QLocaleId::fromName()) are called with environment variables (which, to match anything, must be US-ASCII, so Latin1), so they should at least be ported to QLatin1StringView to avoid the conversion to UTF-16. Since they're already implemented for QStringView, we then have the whole set (UTF-8 can be parsed as L1 here) and can expose the API as QAnyStringView.