mention lint group in default level lint note

[karolzwolak]

Summary

This PR updates lint diagnostics so that default-level notes now mention the lint group they belong to, if any.
Fixes: #65464.

Example

fn main() {
    let x = 5;
}

Before:

= note: `#[warn(unused_variables)]` on by default

After:

= note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default

Unchanged Cases

Messages remain the same when the lint level is explicitly set, e.g.:

• Attribute on the lint #[warn(unused_variables)]:

  note: the lint level is defined here
  LL | #[warn(unused_variables)]
     |        ^^^^^^^^^^^^^^^^
  

• Attribute on the group #[warn(unused)]::

  = note: `#[warn(unused_variables)]` implied by `#[warn(unused)]`
  

• CLI option -W unused:

  = note: `-W unused-variables` implied by `-W unused`
  = help: to override `-W unused` add `#[allow(unused_variables)]`
  

• CLI option -W unused-variables:

  = note: requested on the command line with `-W unused-variables`