No more "transplant" behavior in "let @x = y", gotta box explicitly.

src/test/run-pass/exterior.rs

@@ -10,7 +10,7 @@ fn f(@point p) {
 
 fn main() {
   let point a = rec(x=10, y=11, z=mutable 12);
-  let @point b = a;
+  let @point b = @a;
   check (b.z == 12);
   f(b);
   check (a.z == 12);