Parse non-lifetime binders on closures

[dtolnay]

This is the intersection of rust-lang/rust#97362 with rust-lang/rust#108185.

fn main() {
    let _ = for<T> |_: T| {};
}

error: expected `::`
 --> dev/main.rs:2:20
  |
2 |     let _ = for<T> |_: T| {};
  |                    ^

Syn is expecting :: as if it were something like this:

struct T;

impl T {
    const UNIT: () = ();
}

fn main() {
    let _ = for <T>::UNIT in None {};
}

[dtolnay]

The for <T>::U in v {} syntax used to be valid in old versions of rustc, but since rust-lang/rust#70209 in Rust 1.44 it has been intentionally made an error. So a lookahead after > (to determine whether the next token could be a closure: one of async or move or static or use or |) should not be necessary.