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/query/dropck_outlives.rs

@@ -112,7 +112,7 @@ pub fn trivial_dropck_outlives<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> bool {
         }
 
         ty::Adt(def, _) => {
-            if Some(def.did) == tcx.lang_items().manually_drop() {
+            if Some(def.did()) == tcx.lang_items().manually_drop() {
                 // `ManuallyDrop` never has a dtor.
                 true
             } else {