Fix folly::Future and folly::coro::Future ambiguity in FutureUtil.h
Fix folly::Future and folly::coro::Future ambiguity in FutureUtil.h

Summary: If folly/experimental/coro/Promise.h gets included before folly/experimental/coro/FutureUtil.h, then `toTask(Future<Unit>)` starts referring to `folly::coro::Future` instead of `folly::Future`.

Reviewed By: iahs

Differential Revision: D34583002

fbshipit-source-id: c66758b96cafd17e9cbb04067ec1e6d21027ff23

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly