Prevent calling stack closures through immutable & pointers.

[bblum]

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.

[catamorphism]

Accepted for backwards compatible

[nikomatsakis]

cc #2202

[nikomatsakis]

Closing as dup of #12224 which is more modern and has a better test case.