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_llvm/src/debuginfo/mod.rs

@@ -524,7 +524,7 @@ impl<'ll, 'tcx> DebugInfoMethods<'tcx> for CodegenCx<'ll, 'tcx> {
                             {
                                 Some(type_metadata(cx, impl_self_ty))
                             } else {
-                                Some(namespace::item_namespace(cx, def.did))
+                                Some(namespace::item_namespace(cx, def.did()))
                             }
                         }
                         _ => None,