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_codegen_ssa/src/mir/place.rs

@@ -149,7 +149,7 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
             _ if !field.is_unsized() => return simple(),
             ty::Slice(..) | ty::Str | ty::Foreign(..) => return simple(),
             ty::Adt(def, _) => {
-                if def.repr.packed() {
+                if def.repr().packed() {
                     // FIXME(eddyb) generalize the adjustment when we
                     // start supporting packing to larger alignments.
                     assert_eq!(self.layout.align.abi.bytes(), 1);