Avoid use of std::invoke_result in tag_invoke_result and variants

[msimberg]

This replaces the use of std::invoke_result to compute tag_invoke_result* with a direct use of decltype. At least with libstdc++ std::invoke_result results in three levels of template instantiations. Since we in these cases know that we are only making direct function calls, never member function calls, we don't need the more expensive std::invoke_result. This very slightly improves compile times in DLA-Future.