unused_labels lint does not ignore '_label

[BartMassey]

This code

#![warn(unused_labels)]

fn main() {
    '_unused: loop {
        println!("hello world");
        break;
    }
}

yields this message

warning: unused label
 --> src/main.rs:4:5
  |
4 |     '_unused: loop {
  |     ^^^^^^^^
  |
note: lint level defined here
 --> src/main.rs:1:9
  |
1 | #![warn(unused_labels)]
  |         ^^^^^^^^^^^^^

I think this is not intended behavior: labels beginning with _ should be ignored by this lint, just as identifiers beginning with _ are ignored by similar lints in other contexts.

Right now I think this blocks Issue #66325, since we wouldn't want to start warn-by-default until it is fixed.