Parse unsafe binder types

[dtolnay]

• Tracking issue: Tracking issue for unsafe binder types rust-lang/rust#130516
• Feature gate: #![feature(unsafe_binders)]

fn foo() -> unsafe<'a> &'a () {}

struct Foo {
    x: unsafe<'a> &'a (),
}

struct Bar(unsafe<'a> &'a ());

impl Trait for unsafe<'a> &'a () {}