Add FOLLY_CONSTINIT for C++20's constinit keyword
Add FOLLY_CONSTINIT for C++20's constinit keyword

Summary: This is useful to allow enforcing that things are statically initialized where it is required for correctness.

Reviewed By: jalopezsilva

Differential Revision: D35624220

fbshipit-source-id: 182c96f818da7f03fb442be85bd3c1251382321a

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