Parse `..` pattern in pattern of `let`

[dtolnay]

This needs to be syntactically accepted by consequence of rust-lang/rfcs#2707. Currently rustc can parse it but syn cannot.

#[cfg(any())]
fn main() {
    let .. = 10;
}

error: expected one of: literal, identifier, `::`, `<`, `self`, `Self`, `super`, `crate`, `const`
 --> dev/main.rs:3:12
  |
3 |     let .. = 10;
  |            ^