Add a test that lambdas can't deinitialize upvars

src/test/compile-fail/block-deinitializes-upvar.rs

@@ -0,0 +1,8 @@
+// error-pattern:assigning to upvar
+fn force(f: &block() -> int) -> int { ret f(); }
+fn main() {
+    let x = 5;
+    let f = lambda () -> int { let y = 6; x <- y; ret 7 };
+    assert (force(f) == 7);
+    log x;
+}