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/src/intravisit.rs

@@ -912,7 +912,7 @@ pub fn walk_fn<'v, V: Visitor<'v>>(
 
 pub fn walk_trait_item<'v, V: Visitor<'v>>(visitor: &mut V, trait_item: &'v TraitItem<'v>) {
     // N.B., deliberately force a compilation error if/when new fields are added.
-    let TraitItem { ident, generics, ref defaultness, ref kind, span, def_id: _ } = *trait_item;
+    let TraitItem { ident, generics, ref defaultness, ref kind, span, owner_id: _ } = *trait_item;
     let hir_id = trait_item.hir_id();
     visitor.visit_ident(ident);
     visitor.visit_generics(&generics);
@@ -952,7 +952,7 @@ pub fn walk_trait_item_ref<'v, V: Visitor<'v>>(visitor: &mut V, trait_item_ref:
 pub fn walk_impl_item<'v, V: Visitor<'v>>(visitor: &mut V, impl_item: &'v ImplItem<'v>) {
     // N.B., deliberately force a compilation error if/when new fields are added.
     let ImplItem {
-        def_id: _,
+        owner_id: _,
         ident,
         ref generics,
         ref kind,