Flow analysis: consider adding tracking of "causes" of type promotions

[stereotype441]

This would enable features like #38773.

[stereotype441]

I'm removing this from the "Flow analysis" epic and the "NNBD Support" release, since it's not worth doing in and of itself; it's only worth doing as part of #38773.

[stereotype441]

Closing this issue since its only reason for existence is to facilitate #38773, and it makes sense to work on both as a single task.