constexpr QLatin1String

Currently QLatin1String can not be constructed as constexpr string due to usage of strlen() in the constructor. Changing the constructor like in the workaround below overcomes that limitation:

struct ConstLatin1String : public QLatin1String
{
 constexpr ConstLatin1String(const char* const s) : QLatin1String(s, static_cast<int>(std::char_traits<char>::length(s))) {}
};