fix #3105 - Compile-time error when mixing named argument with automatic indexing

[rbrich]

Fixes #3105

Looks like this is enough to fix the issue.

I compared the runtime checking with compile-time checking and adjusted the compile-time version to mirror the runtime one.

The runtime check is this (second function for named args):

    FMT_CONSTEXPR auto on_arg_id(int id) -> int {
      return parse_context.check_arg_id(id), id;
    }
    FMT_CONSTEXPR auto on_arg_id(basic_string_view<Char> id) -> int {
      int arg_id = context.arg_id(id);
      if (arg_id < 0) on_error("argument not found");
      return arg_id;
    }

It returns the arg index directly without running it through the check_arg_id, unlike the indexed arg check.

[vitaut]

Thank you!