Unannotated async function should be inferred as `Awaitable[Any]`

[Dreamsorcerer]

An async function with no return type declaration, like:
async def foo():

This is inferred as returning Any, but should be inferred as returning Awaitable[Any]. The result of this has caused us to miss a bunch of unused awaitable warnings via incomplete stubs.

[JelleZijlstra]

Probably even Coroutine[Any, Any, Any]

[ritikBhandari]

so we've to change its return type to Awaitable[Any] right?