[clangd] Fix compile error after 20b69af7

Some of the buildbots were failing due to what seems to be them using a non c++14 compilant std::string implementation.
Since c++14 std::basic_string::append(const basic_string, size_t, size_t) has a defaulted 3rd paramater, but some of the build bots were reporting that it wasn't defaulted in their implementation.