Promise<T>::makeEmpty() and Future<T>::makeEmpty()

Promise<T>::makeEmpty() and Future<T>::makeEmpty()

Summary:
[Folly] `Promise<T>::makeEmpty()` and `Future<T>::makeEmpty()`.

These can currently be done by creating, and then moving away from, regular promises and futures. But that allocates; this is semantically equivalent but more efficient.

Differential Revision: D5368339

fbshipit-source-id: de054cfc75c701c5d39aac64d9a2949cd34b1896