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_hir_analysis/src/lib.rs

@@ -381,7 +381,7 @@ fn check_start_fn_ty(tcx: TyCtxt<'_>, start_def_id: DefId) {
                         error = true;
                     }
                     if let hir::IsAsync::Async = sig.header.asyncness {
-                        let span = tcx.def_span(it.def_id);
+                        let span = tcx.def_span(it.owner_id);
                         struct_span_err!(
                             tcx.sess,
                             span,