Summary:
[Folly] Let `assume` be invocable anytime, including before and after `main` and in signal handlers, by switching from `CHECK` to `FOLLY_SAFE_CHECK`.

Requires further minimizing the deps of `FOLLY_SAFE_CHECK`.

Reviewed By: ot, Orvid, luciang

Differential Revision: D6636035

fbshipit-source-id: 026b15ea92ba37acf9b11d714d834b79da80936a