Summary:
To preserve void type in roundtrip
Doing this generically is pretty gross (you need to check if awaiter_type<decltype(co_viaIfAsync({}, SemiAwaitable))> has a member await_resume_try), while the cost of adding explicit overloads is low as there are only two types we care about, so I chose to do it that way.

Reviewed By: yfeldblum

Differential Revision: D23407270

fbshipit-source-id: cc952e2d8fa4297dbe363e22e99192fdf858134f