fix: support optional<T> with format_as(T) (#3713)

Formatting a std::optional<T> where T had a custom format_as(T) function failed to compile with clang,
due to set_debug_format being hidden by private inheritance. This fix makes the function available through a using clause.

include/fmt/format.h

@@ -4054,6 +4054,7 @@ struct formatter<T, Char, enable_if_t<detail::has_format_as<T>::value>>
     : private formatter<detail::format_as_t<T>, Char> {
   using base = formatter<detail::format_as_t<T>, Char>;
   using base::parse;
+  using base::set_debug_format;
 
   template <typename FormatContext>
   auto format(const T& value, FormatContext& ctx) const -> decltype(ctx.out()) {