#[must_use] does not warn for unused must_use tuple components

``` rust
#[must_use]
struct MU;

fn a() -> (u8, MU) {
    (0, MU)
}

pub fn main() {
    a(); // does not warn
    a().1; // warns
    a().0; // does not warn
}
```

I believe it should warn in all three cases.
