Auto merge of #41911 - michaelwoerister:querify_trait_def, r=nikomatsakis

Auto merge of #41911 - michaelwoerister:querify_trait_def, r=nikomatsakis

Remove interior mutability from TraitDef by turning fields into queries

This PR gets rid of anything `std::cell` in `TraitDef` by
- moving the global list of trait impls from `TraitDef` into a query,
- moving the list of trait impls relevent for some self-type from `TraitDef` into a query
- moving the specialization graph of trait impls into a query, and
- moving `TraitDef::object_safety` into a query.

I really like how querifying things not only helps with incremental compilation and on-demand, but also just plain makes the code cleaner `:)`

There are also some smaller fixes in the PR. Commits can be reviewed separately.

r? @eddyb or @nikomatsakis