ICE when using associated const in enum discriminant

``` rust
#![feature(associated_consts)]
enum NonPortable2<T: Trait> {
    X = Trait::Number,
    Y = 0,
}
trait Trait {
    const Number: usize = 1;
}
fn main() {}
```

> thread 'rustc' panicked at 'No def'n found for DefId { krate: 0, node: DefIndex(9) => Trait::Number } in tcx.impl_or_trait_items', ../src/librustc/middle/ty/mod.rs:1813
