Reference to const that is a reference implicitly flattens

``` rust
fn foo() -> &'static i32 {
    const F: &'static i32 = &3;
    &F
}

fn main() {
    println!("{}", foo());
}
```

This compiles and prints the right value, as does the same program with the `&` in front of `F` removed.

cc @eddyb probably related to the const rvalue promotions change.
