Missing diagnostic when capturing self in deinit Task

[rnapier]

          Seems feasible. The overall trouble with static analysis is that it's semi-common (especially when using AppKit/UIKit) to pass `self` out to other methods in deinit that then do Mysterious Things to it, and you're essentially pinky promising that it doesn't _really_ escape. That means the compiler can't be too strict about it. But capturing `self` with a `Task` or async dispatch is pretty much guaranteed to be wrong, so a warning should be doable for those specific cases.

Originally posted by @mikeash in #62191 (comment)