Narrow types for walrus assignment in if statements

[hauntsaninja]

Fixes #7313
Fixes #7316

[hauntsaninja]

Let me know if there's a better way to handle the partial types that can result! Or if there are more tests I should be adding. There's a lot that I don't know :-)

I guess one thing I was wondering if there's a way to improve the hint resulting from the partial type (see line 296 on test-data/unit/check-python38.test). Worried that it might be hard for users to realise that you would have to put the annotation on another line.

[JelleZijlstra]

I doubt that a lot of users are going to write if (x := []): (why would you do that except to confuse a typechecker?), so I don't think it's worth it to invest too much in improving the error message. We can revisit that if we get user complaints.

[TH3CHARLie]

Will you have an interest in testing against #7313 to see if the PR also happens to fix that issue? (Although I am not sure at all whether these issues come from the same source)

[hauntsaninja]

Yes, it fixes #7313 too (updated the PR to mention that). A test exists for that issue on line 269 of test-data/unit/check-python38.test.

Looking through other issues, it does not fix #8236, but I can maybe work on that (in a separate PR).

[msullivan]

This seems good to me and is really important since this sort of thing is the killer feature of assignment expressions.