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_borrowck/src/diagnostics/move_errors.rs

@@ -388,7 +388,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         };
         let ty = move_place.ty(self.body, self.infcx.tcx).ty;
         let def_id = match *ty.kind() {
-            ty::Adt(self_def, _) => self_def.did,
+            ty::Adt(self_def, _) => self_def.did(),
             ty::Foreign(def_id)
             | ty::FnDef(def_id, _)
             | ty::Closure(def_id, _)