Use edition flag for 2phase test

The compiler now rejects one of the examples with #![feature(nll)]

Author: matthewjasper

tests/run-pass/2phase.rs

@@ -1,4 +1,4 @@
-#![feature(nll)]
+// edition:2018
 
 trait S: Sized {
     fn tpb(&mut self, _s: Self) {}
@@ -54,6 +54,8 @@ fn with_interior_mutability() {
 
     let mut x = Cell::new(1);
     let l = &x;
+
+    #[allow(unknown_lints, mutable_borrow_reservation_conflict)]
     x
         .do_the_thing({
             x.set(3);