Auto merge of #44696 - michaelwoerister:fingerprints-in-dep-graph-3, r=nikomatsakis

Auto merge of #44696 - michaelwoerister:fingerprints-in-dep-graph-3, r=nikomatsakis

incr.comp.: Move task result fingerprinting into DepGraph.

This PR
- makes the DepGraph store all `Fingerprints` of task results,
- allows `DepNode` to be marked as input nodes,
- makes HIR node hashing use the regular fingerprinting infrastructure,
- removes the now unused `IncrementalHashesMap`, and
- makes sure that `traits_in_scope_map` fingerprints are stable.

r? @nikomatsakis
cc @alexcrichton