Two more testcases for overwriting already-live box-like fields in structures, without leaking.

src/test/run-pass/box-in-tup.rs

@@ -0,0 +1,4 @@
+fn main() {
+  let tup(mutable @int) i = tup(mutable @10);
+  i._0 = @11;
+}

src/test/run-pass/vec-in-tup.rs

@@ -0,0 +1,4 @@
+fn main() {
+  let tup(mutable vec[int]) i = tup(mutable vec(1,2,3));
+  i._0 = vec(4,5,6);
+}