Auto merge of #42537 - michaelwoerister:tcx-for-dep-node, r=nikomatsakis

incr.comp.: Make DepNode `Copy` and valid across compilation sessions

This PR moves `DepNode` to a representation that does not need retracing and thus simplifies comparing dep-graphs from different compilation sessions. The code also gets a lot simpler in many places, since we don't need the generic parameter on `DepNode` anymore.  See https://github.com/rust-lang/rust/issues/42294 for details.

~~NOTE: Only the last commit of this is new, the rest is already reviewed in https://github.com/rust-lang/rust/pull/42504.~~

This PR is almost done but there are some things I still want to do:
- [x] Add some module-level documentation to `dep_node.rs`, explaining especially what the `define_dep_nodes!()` macro is about.
- [x] Do another pass over the dep-graph loading logic. I suspect that we can get rid of building the `edges` map and also use arrays instead of hash maps in some places.

cc @rust-lang/compiler
r? @nikomatsakis

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