Parse unnamed enum variant syntax

[dtolnay]

• RFC: Open up your enum with an unnamed variant rust-lang/rfcs#3894
• Tracking Issue for Unnamed Enum Variants (Open Enums) rust-lang/rust#156628

#[repr(u8)]
enum Color {
    Red = 0,
    Green = 1,
    _ = ..,  // All other discriminants in `u8` are assigned to unnamed variants
}