Handle union types when binding self

[ilevkivskyi]

Currently we only bind self if the type is callable, but we actually should do this for all callable items in a union.

This use case is probably quite niche (since adding an annotation makes a variable an instance variable, and we rarely infer unions). I found it when looking at checkmember-related issues it was easy to handle it. I also use this opportunity to refactor and add comments to analyze_var().

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

spark (https://github.com/apache/spark)
+ python/pyspark/util.py:545: error: Unused "type: ignore[union-attr]" comment  [unused-ignore]
+ python/pyspark/util.py:584: error: Unused "type: ignore[union-attr]" comment  [unused-ignore]