Back out "Revert D18927220: if_constexpr for C++14" (#37792)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37792
Original commit changeset: a1b8755a2790
ghstack-source-id: 103609715
Test Plan: waitforsandcastle
Differential Revision: D21389755
fbshipit-source-id: 1a3c74295dbfbf07fe225be9bcd47d11e31a20fa
