or_patterns: unused_parens lints erroneously on _a @ (0 | 1)

[Centril]

#![feature(or_patterns)]
#![allow(incomplete_features)]

fn foo() {
    match 0 {
        _a @ (0 | 1) => {}
        _ => {}
    }

    let () = 0; // Prevent the ICE in MIR.
}

results in:

warning: unnecessary parentheses around pattern
 --> src/lib.rs:6:14
  |
6 |         _a @ (0 | 1) => {}
  |              ^^^^^^^ help: remove these parentheses
  |
  = note: `#[warn(unused_parens)]` on by default

changing things into _a @ (0 | 1) will correctly refuse to parse.

cc @dlrobertson #54883

[Centril]

Keep-sake: #55342.