Cut extra template instantiations from Unit traits
Cut extra template instantiations from Unit traits

Summary: [Folly] Cut extra template instantiations from `Unit` traits - namely, `std::conditional` and `std::is_same`. The behavior can be done directly, while each extra template instantiations imposes some small compile-time cost.

Reviewed By: Orvid

Differential Revision: D8178033

fbshipit-source-id: 22ba5ae810e2bca616e7db332c1f635e2520f3a7

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift