An async method's return annotation (if there is one) must be "Awaitable"

Summary: This takes the first step for improving return type enforcement for async
functions. We enforce that if there is a return type annotation, it must be
"Awaitable" or "Awaitable<T>".

This diff also adds some missing plumbing for return annotations for
closures and lambdas.

Reviewed By: @elgenie

Differential Revision: D1566028