closure upvars disable `unused_assignments` lint

fn main() {
    let mut overwritten = false;
    let _ = || overwritten = true;
}

does not trigger any lints while

fn main() {
    let mut overwritten = false;
    if false {
        overwritten = true;
    }
}

results in

warning: variable `overwritten` is assigned to, but never used
 --> src/main.rs:2:13
  |
2 |     let mut overwritten = false;
  |             ^^^^^^^^^^^
  |
  = note: consider using `_overwritten` instead
  = note: `#[warn(unused_variables)]` on by default

The unused_variables and lint should look into nested closures here.

Reproduce the two Rust examples from the issue and compare the behavior of the unused_variables and unused_assignments lints around nested closures. Locate their compiler implementations and existing lint tests, then add coverage showing that captured assignments are diagnosed; done means the closure example emits the appropriate lint without regressing the conditional-assignment case.