Parse non-lifetime binders

[dtolnay]

As seen in https://github.com/rust-lang/rust/blob/b9535c0b7d64290b27c4a116262402cd12f77833/tests/rustdoc/non_lifetime_binders.rs.

fn foo() where for<'a, T> &'a Wrapper<T>: Trait {}

https://github.com/rust-lang/rust/pull/108335