Named enum fields are not named

```rust
enum Foo {
  A { x: u32, y: u32 },
  B { y: u32, z: u32 },
}
```
should result in
```rust
struct FooRef<'a> {
  discriminant: &'a FooDiscriminant,
  x: &'a FooFieldX,
  y: &'a FooFieldY,
  z: FooFieldZ,
}
```

Right now we instead get `field0` and `field1` corresponding to `union { A::x, B::y }` and `union { A::y, B::z }`.