Extend contextmanager to asynccontextmanager - #11352

Closes  python#9922

### Description

Extending the existing special case for `contextlib.contextmanager` to `contextlib.asynccontextmanager`. When used with `TypeVar` gives much better type.


## Test Plan

I hope I have included the unit test you need. My minimal reproduction is:

```
from contextlib import asynccontextmanager
from typing import AsyncGenerator, Type, TypeVar

R = TypeVar("R")


@asynccontextmanager
async def foo(
    cls: Type[R],
) -> AsyncGenerator[R, None]:
    yield cls()


async def bar() -> int:
    async with foo(int) as foo_int:
        return (
            foo_int  # error: Incompatible return value type (got "R", expected "int")
        )
```