Save used-trait-import sets to the incremental state and invalidate when reloading if deps have changed

[nikomatsakis]

Background: See the general plan for skipping typeck. Part of the roadmap issue on incremental compilation.

Once #39495 is done, we need to save the contents of the used_trait_imports map to the incremental state. Since the values are basically just def-ids, we have all the necessary infrastructure. When re-loading, if we find that a UsedTraitImport(X) node has been marked as dirty, we will remove the corresponding value from the map. For the clean values, we can reload their entries and store them into the map.

[nikomatsakis]

Closing this, as we've been adjusting plan and I no longer this this is a logical next step.