support if-constexpr macros on MSVC
support if-constexpr macros on MSVC

Summary: change the macros to work on MSVC

Reviewed By: yfeldblum

Differential Revision: D14942488

fbshipit-source-id: 12d6814230bd9b92b54603cd711d03cd8b5f8086

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly