Improve AdtDef interning.

This commit makes `AdtDef` use `Interned`. Much the commit is tedious
changes to introduce getter functions. The interesting changes are in
`compiler/rustc_middle/src/ty/adt.rs`.

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -540,7 +540,7 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
 
                 ty::Adt(def, substs) => {
                     // WfNominalType
-                    let obligations = self.nominal_obligations(def.did, substs);
+                    let obligations = self.nominal_obligations(def.did(), substs);
                     self.out.extend(obligations);
                 }