shadow_unrelated breaks when intermediary bindings are used

Playground:

pub fn foo(a: i32) {
    let b = 3 * (3 + 1) / 2;
    let c = (f64::from(a) / f64::from(b)) as i32;

    let a = c + b;

    let _ = a;
}

produces

error: `a` is shadowed by `c + b`
 --> src/main.rs:8:9
  |
8 |     let a = c + b;
  |         ^
  |
note: lint level defined here
 --> src/main.rs:2:44
  |
2 | #![cfg_attr(feature = "cargo-clippy", deny(clippy::shadow_unrelated))]
  |                                            ^^^^^^^^^^^^^^^^^^^^^^^^
note: initialization happens here
 --> src/main.rs:8:13
  |
8 |     let a = c + b;
  |             ^^^^^
note: previous binding is here
 --> src/main.rs:4:12
  |
4 | pub fn foo(a: i32) {

The lint incorrectly errors that the new binding a is unrelated to the function argument a, but this is incorrect since these two bindings are related via c.

Reproduce the example in the linked Rust Playground and begin at the shadow_unrelated lint entry point. Trace how bindings connected through c are classified, then verify that the shown a binding is no longer reported as unrelated.