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 #42294 for details.

~~NOTE: Only the last commit of this is new, the rest is already reviewed in #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

src/librustc/dep_graph/debug.rs

@@ -12,7 +12,6 @@
 
 use super::dep_node::DepNode;
 use std::error::Error;
-use std::fmt::Debug;
 
 /// A dep-node filter goes from a user-defined string to a query over
 /// nodes. Right now the format is like this:
@@ -39,7 +38,7 @@ impl DepNodeFilter {
     }
 
     /// Tests whether `node` meets the filter, returning true if so.
-    pub fn test<D: Clone + Debug>(&self, node: &DepNode<D>) -> bool {
+    pub fn test(&self, node: &DepNode) -> bool {
         let debug_str = format!("{:?}", node);
         self.text.split("&")
                  .map(|s| s.trim())
@@ -67,10 +66,10 @@ impl EdgeFilter {
         }
     }
 
-    pub fn test<D: Clone + Debug>(&self,
-                                  source: &DepNode<D>,
-                                  target: &DepNode<D>)
-                                  -> bool {
+    pub fn test(&self,
+                source: &DepNode,
+                target: &DepNode)
+                -> bool {
         self.source.test(source) && self.target.test(target)
     }
 }