Rollup merge of #110960 - lukas-code:unused-mut, r=compiler-errors

fix false negative for `unused_mut`

fixes rust-lang/rust#110849

We want to avoid double diagnostics for code like this, but only if an error actually occurs:
```rust
fn main() {
    let mut x: (i32, i32);
    x.0 = 1;
}
```

The first commit fixes the lint and the second one removes all the unused `mut`s it found.

Author: matthiaskrgr