`fn foo<T:?>() {}` compiles despite the weird trait bound

Both of the following compile fine <https://is.gd/k1iXTl>:
```rust
fn foo<T:?>() {}

use std::error::Error;
fn bar<E>() -> Result<(),E>
    where E : Error + ?
{
    Ok(())
}
```

I would have expected that a lonely `?` (without a trait name) in a bound was an error.

(Thanks @tomaka on IRC for the simpler repro.)