Add support for back_insert_iterator

test/custom-formatter-test.cc

@@ -36,7 +36,7 @@ class CustomArgFormatter :
 std::string custom_vformat(fmt::string_view format_str, fmt::format_args args) {
   fmt::memory_buffer buffer;
   // Pass custom argument formatter as a template arg to vwrite.
-  fmt::vformat_to<CustomArgFormatter>(buffer, format_str, args);
+  fmt::do_vformat_to<CustomArgFormatter>(buffer, format_str, args);
   return std::string(buffer.data(), buffer.size());
 }