Pretty printer doesn't preserve built-in trait bounds on trait objects

``` rust
fn foo(x: ~Send:Freeze) -> ~Send:Freeze { x }

fn main() {
    let x: ~Send:Freeze;

    ~1 as ~Send:Freeze;
}
```

pretty prints to

``` rust
fn foo(x: ~Send) -> ~Send { x }

fn main() {
    let x: ~Send;

    ~1 as ~Send;
}
```

(I xfail-pretty'd a test in #9225 because of this.)
