Signature help may include same-named methods that are not on the declaring class hierarchy

[rgrunber]

See #3052 (comment)

Background
The main issue that #3052 solved was that when signature help is done on some method invocation, the overloads (if any) are only discovered for the inherited classes of the declaring class that declares the method. This completely misses the subclasses. Trying to find the subclasses is expensive, which is probably why we never did it. That's why we got rid of hierarchy logic.

However, now we don't have a way to catch same-named methods that don't have anything to do with the hierarchy. If you look at the example, it's a pretty rare since the casting in the example is needed to reproduce the issue.