Fix fulldeps-ui tests

Author: estebank

tests/ui-fulldeps/dropck-tarena-cycle-checked.stderr

@@ -1,6 +1,8 @@
 error[E0597]: `arena` does not live long enough
   --> $DIR/dropck-tarena-cycle-checked.rs:116:7
    |
+LL |     let arena = TypedArena::default();
+   |         ----- binding `arena` declared here
 LL |     f(&arena);
    |       ^^^^^^ borrowed value does not live long enough
 LL | }

tests/ui-fulldeps/dropck-tarena-unsound-drop.stderr

@@ -1,6 +1,8 @@
 error[E0597]: `arena` does not live long enough
   --> $DIR/dropck-tarena-unsound-drop.rs:41:7
    |
+LL |     let arena: TypedArena<C> = TypedArena::default();
+   |         ----- binding `arena` declared here
 LL |     f(&arena);
    |       ^^^^^^ borrowed value does not live long enough
 LL | }