Improve multiline reporting

Current multiline error/notice reporting is hacky, e.g.
https://github.com/python/mypy/blob/c1fb57d38116adf6c1c1ea5c4b998cae597ec156/mypy/messages.py#L1161-L1169

Notice how

• we call notice multiple times even as it relates to the same core issue
• we pass allow_dups=True to suppress deduplication of, say, the same type
• we mistakenly(?) dedup the "Superclass:" and "Subclass:" notices
• by always doing allow_dups=False, we can effectively dedup when it would make sense

I think it'll be better if the extra lines could be part of the error, also reducing code duplication (we end up passing a lot of the same parameters multiple times). Is there a reason why we didn't? Is it a pattern elsewhere in the code?

Start in mypy/messages.py at the linked lines 1161-1169 and trace the related notice and deduplication behavior. Look for other multiline reporting patterns and determine how extra lines could belong to one error without incorrect deduplication. Done should reduce repeated parameter passing while preserving the intended distinction between related notices.