Rename some OwnerId fields.

spastorino noticed some silly expressions like `item_id.def_id.def_id`.

This commit renames several `def_id: OwnerId` fields as `owner_id`, so
those expressions become `item_id.owner_id.def_id`.

`item_id.owner_id.local_def_id` would be even clearer, but the use of
`def_id` for values of type `LocalDefId` is *very* widespread, so I left
that alone.

compiler/rustc_passes/src/weak_lang_items.rs

@@ -34,7 +34,7 @@ pub fn check_crate<'tcx>(tcx: TyCtxt<'tcx>, items: &mut lang_items::LanguageItem
                     items.missing.push(item);
                 }
             } else {
-                let span = tcx.def_span(id.def_id);
+                let span = tcx.def_span(id.owner_id);
                 tcx.sess.emit_err(UnknownExternLangItem { span, lang_item });
             }
         }