Fix fallback to runtime API from compile-time API

[alexezeder]

Right now, when {fmt} fails to prepare compiled format for format string (using FMT_COMPILE or _cf) it also fails to report about that clearly. Here is an example - https://godbolt.org/z/G1ETdc.

There were some attempts to make the error a bit clearer, but as it turned out {fmt} should fall back to the runtime API in those cases. This PR adds that fallback.

Some points for the PR:

• test added
• there is a bit of code duplication introduced, because I decided that this code is clearer:

  #ifdef __cpp_if_constexpr
    if constexpr (std::is_same<remove_cvref_t<decltype(compiled)>,
                               detail::unknown_format>()) {
      return format(static_cast<basic_string_view<typename S::char_type>>(S()),
                    std::forward<Args>(args)...);
    } else {
      return format(compiled, std::forward<Args>(args)...);
    }
  #else
    return format(compiled, std::forward<Args>(args)...);
  #endif

  then this one:

  #ifdef __cpp_if_constexpr
    if constexpr (std::is_same<remove_cvref_t<decltype(compiled)>,
                               detail::unknown_format>()) {
      return format(static_cast<basic_string_view<typename S::char_type>>(S()),
                    std::forward<Args>(args)...);
    } else {
  #endif
      return format(compiled, std::forward<Args>(args)...);
  #ifdef __cpp_if_constexpr
    }
  #endif

[vitaut]

Thanks for the PR. Looks good but I don't think we should provide a way to disable the fallback.

[vitaut]

Can we replace all of these by forwarding S to format_to call below which will handle fallback?

[alexezeder]

Yes, we can. It will create one more template instantiation, but it shouldn't be tough.
So, it's done.

[vitaut]

Thank you!