Summary:
[Folly] An `unsorted` tag.

For contexts in which it is desired explicitly to allow, as an optimization, the passing of unsorted containers, where such would ordinarily be assumed to be a pessimization or assumed to be incorrect.

Reviewed By: igorsugak

Differential Revision: D6782860

fbshipit-source-id: 0bc7002306497e8bb0a62c4589d4f243da062ff8