Parse supertrait auto impl syntax

[dtolnay]

• Supertrait Auto-impl rust-lang/rfcs#3851
• Tracking Issue for Supertrait auto impl rust-lang/rust#149556

trait Subtrait2: Supertrait2 {
    auto impl Supertrait2 {
        type Type1 = Self;
        type Type2 = ();
    }
}