If we're building on C++17, use actual "if constexpr" (#38154)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38154
This should give better error messages and shorter stack traces on C++17 builds (e.g. fbcode)
ghstack-source-id: 103775564
Test Plan: waitforsandcastle
Differential Revision: D21483327
fbshipit-source-id: 184d1f9c0543bf43dc9713fa97fcc5955e7be319
