Implement changes to strong mode top level inference

[leafpetersen]

We have an informal spec for top level inference for strong mode here: #28218 . The proposal is ready for feedback from implementers and other team members, and subsequent implementation in the analyzer. This is the tracking bug for the analyzer implementation.

[jmesserly]

yay!!! 🎉

[kevmoo]

@leafpetersen critical for 1.22? Trying to prioritize...

[leafpetersen]

Lower priority than things that affect non-strong mode (especially syntax).

[mit-mit]

Punting to 1.23 after priority discussion