Track location for atomic new

This doesn't _really_ add any information, since the mut location is the
same as the "crated" location. However, when presented with an error
with no `mut` location, the user may erroneously believe that there's
some mutation they haven't been told about, when the race is really
on the construction of the item.

src/rt/atomic.rs

@@ -358,6 +358,7 @@ impl State {
         };
 
         // All subsequent accesses must happen-after.
+        state.unsync_mut_locations.track(location, &threads);
         state.track_unsync_mut(threads);
 
         // Store the initial thread