gh-135736: Fix interaction between TaskGroup and aclose() (#154649)

gh-135736: Fix interaction between TaskGroup and aclose() (#154649)

Currently if you have a generator like:
```
async def test():
    async with asyncio.TaskGroup() as tg:
        async for x in whatever():
            yield x
```

If aclose() is called on the generator and GeneratorExit is raised,
the TaskGroup's __aexit__ will raise a BaseExceptionGroup, and that
will be raised from the aclose() call instead of it being swallowed.

Fix this by raising GeneratorExit from __aexit__ when:
1. The body of task group raised it
2. No subtasks raised exceptions

This makes GeneratorExit work without swallowing other exceptions
(like it would if we just added it to _is_base_error).