Only store a LocalDefId in hir::ImplItem.

compiler/rustc_middle/src/ty/error.rs

@@ -836,7 +836,7 @@ fn foo(&self) -> Self::T { String::new() }
             })) => {
                 for item in &items[..] {
                     if let hir::AssocItemKind::Type = item.kind {
-                        if self.type_of(self.hir().local_def_id(item.id.hir_id)) == found {
+                        if self.type_of(item.id.def_id) == found {
                             db.span_label(item.span, "expected this associated type");
                             return true;
                         }