ICE with C generic enum

http://is.gd/DOkUCJ Playground with backtrace

``` rust
#[repr(C)]
pub enum CPOption<T> {
    PSome(T),
}

fn main() {
  println!("sizeof CPOption<i32> {}", std::mem::size_of::<CPOption<i32>>());
}
```

Versions affected: stable, beta, nightly.
