An `enum` with a discriminant must specify a `#[repr(inttype)]`.

A `#[repr(inttype)]` must be provided on an `enum` if it has a non-unit
variant with a discriminant, or where there are both unit variants with
discriminants and non-unit variants. This restriction ensures that there
is a well-defined way to extract a variant's discriminant from a value;
for instance:

```
#![feature(arbitrary_enum_discriminant)]

#[repr(u8)]
enum Enum {
    Unit = 3,
    Tuple(u16) = 2,
    Struct {
        a: u8,
        b: u16,
    } = 1,
}

fn discriminant(v : &Enum) -> u8 {
    unsafe { *(v as *const Enum as *const u8) }
}

assert_eq!(3, discriminant(&Enum::Unit));
assert_eq!(2, discriminant(&Enum::Tuple(5)));
assert_eq!(1, discriminant(&Enum::Struct{a: 7, b: 11}));
```