Anonymous function with upvar trips assert in trans

```
fn main() {
    let x = 3;
    (fn() {
        assert x == 3;
    })();
}
```

Result:

```
rust: upcall fail 'Assertion cx.fcx.llupvars.contains_key(did.node) failed', ../src/comp/middle/trans.rs:2688
rust: domain main @0x7fe932819a00 root task failed
```

Expected?

```
error: attempted dynamic environment-capture
```
