Summary: [Folly] Test `constexpr_strlen`, `constexpr_strcmp` fallback implementations alongside the main implementations, since the main implementations may fall back to the fallback implementations on some platforms but not on others, and we may only be testing on the other platforms.

Differential Revision: D23432301

fbshipit-source-id: 271705cb8c2b1ab6d11c1be20e2c3023cacab583