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.

clippy_utils/src/lib.rs

@@ -2281,7 +2281,7 @@ fn with_test_item_names(tcx: TyCtxt<'_>, module: LocalDefId, f: impl Fn(&[Symbol
         Entry::Vacant(entry) => {
             let mut names = Vec::new();
             for id in tcx.hir().module_items(module) {
-                if matches!(tcx.def_kind(id.def_id), DefKind::Const)
+                if matches!(tcx.def_kind(id.owner_id), DefKind::Const)
                     && let item = tcx.hir().item(id)
                     && let ItemKind::Const(ty, _body) = item.kind {
                     if let TyKind::Path(QPath::Resolved(_, path)) = ty.kind {