Summary: as title. Disabling -Wconversion since it can be triggered pretty easily. We've run into this a lot in cachelib code. Until now, we've manually disabled -Wconversion around the header for folly/Format.h in our own code. But I think it's much easier just to disable it inside Format-inl.h itself.

Reviewed By: yfeldblum

Differential Revision: D14471124

fbshipit-source-id: df6bcbd57cbbdb0e6a762eaa5d11548c9e614e1b