Take advantage of C++17 in scalar_type_util.h

Pull Request resolved: #6968

I generated a big ugly table because we couldn't make promoteTypes constexpr before we had C++17. Now we have C++17.
ghstack-source-id: 254805946

Differential Revision: [D66181946](https://our.internmc.facebook.com/intern/diff/D66181946/)

Author: swolchok