Fix typechecking for async generators

[yangdanny97]

Fixes #10534

This PR fixes a bug in typechecking asynchronous generators.

Mypy currently typechecks a generator/comprehension as AsyncGenerator if the leftmost expression contains await, or if it contains an async for.

However, there are other situations where we should get async generator: If there is an await expression in any of the conditions or in any sequence except for the leftmost one, the generator/comprehension should also be typechecked as AsyncGenerator.

I've implemented this change in Mypy and added a test case to assert this behavior. If I enter the test cases into a regular repl, I can confirm that the runtime representation is generator/async_generator as the test case expects.

According to the language reference:

If a comprehension contains either async for clauses or await expressions or other asynchronous comprehensions it is called an asynchronous comprehension.

Confusingly, the documentation itself is actually not quite correct either, as pointed out in python/cpython#114104

Alongside this change, I've made a PR to update the docs to be more precise: python/cpython#121175 has more details.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[hauntsaninja]

Nice, thank you!