let-else: linting for `unused` bindings is not handled correctly

[nagisa]

#![feature(let_else)]

fn example_let_else(value: Option<String>) {
    #[allow(unused)]
    let banana = 1;
    #[allow(unused)]
    let Some(chaenomeles) = value else { return };
}

here banana will not be linted against as an unused variable, but chaenomeleswill be:

warning: unused variable: `chaenomeles`
 --> src/lib.rs:7:14
  |
7 |     let Some(chaenomeles) = value else { return };
  |              ^^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_chaenomeles`
  |
  = note: `#[warn(unused_variables)]` on by default

playground

cc #87335