ICE when calling static and static mut function pointers.

[brendanzab]

fn f(x: int) -> int { x }

static mut F: extern fn(x: int) -> int = f;

fn main() {
    unsafe { F(1) }
}

error: internal compiler error: Cannot translate def def_static({crate: 0, node: 23}, true) to a callable thing!

Seems to have been resolved a while ago (#4523), but is still occurring. @Aatch has said this should be an easy fix, it just needs to be done. This is important for the OpengGL function loader.