Prevent calling stack closures through immutable & pointers.

This is the last hole left over in the "case of the recurring closure" problem which was mostly but not quite fixed by #7363.

This program should be illegal (it isn't itself soundness-breaking, but isn't far from it):

```
fn bar(x: & &fn(), y: & &fn()) { (*x)(); (*y)(); }
fn foo(x: &fn()) { bar(&x, &x) }
fn main() {
        do foo || {}
}
```

Nominating backwards-compatible.
