Provide users with a way to mark uses as irrelevant for unused code lints

See https://github.com/tokio-rs/tracing/issues/2209 for a concrete bug this would fix.

This is related to Debug and Clone impls where we already have such a special case. I would like to generalize this to a #[diagnostics::] attribute usable on stable.

#[derive(Debug)]
pub struct Foo {
    field: u32,
}

pub fn bar(x: Foo) {
    println!("{x:?}");
}

warning: field `field` is never read
 --> src/lib.rs:3:5
  |
2 | pub struct Foo {
  |            --- field in this struct
3 |     field: u32,
  |     ^^^^^
  |
  = note: `Foo` has a derived impl for the trait `Debug`, but this is intentionally ignored during dead code analysis
  = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default