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_save_analysis/src/lib.rs

@@ -554,7 +554,7 @@ impl<'tcx> SaveContext<'tcx> {
                     Some(Data::RefData(Ref {
                         kind: RefKind::Type,
                         span,
-                        ref_id: id_from_def_id(def.did),
+                        ref_id: id_from_def_id(def.did()),
                     }))
                 }
                 _ => {