mypy does not complain about async functions that do not return values #12837

Found as part of #12834

In the following, you'd expect mypy to complain about assigning to await async_nothing, like it does with the synchronous case. Note that mypy does in fact complain about this using the old asyncio.coroutine yield from coroutines, see testErrorAssigningCoroutineThatDontReturn (prior to #12834).

def nothing() -> None:
    ...

def foo() -> None:
    x = nothing()  # E: "nothing" does not return a value

async def async_nothing() -> None:
    ...

async def async_foo() -> None:
    x = await async_nothing()