don't emit `unused_results` lint for tuples of "trivial" types
r? @jdonszelmann
Fixesrust-lang/rust#153144.
So it turns out rust-lang/rust#153018 had a sneaky behavior change in the way tuples are handled and the old behavior wasn't tested.
Consider these tuples:
```rust
((), ());
((), 1);
```
Neither of them is `must_use`, so they are potential candidates for `unused_results`. So the question is whatever said tuples are considered "trivial" and thus if they end up emitting `unused_results` or not.
Here is a comparison table between PRs:
<table>
<tr><td>stable</td><td>After #153018</td><td>After this PR</td></tr><tr><td>
```rust
((), ()); // trivial
((), 1); // trivial
```
</td>
<td>
```rust
((), ()); //~ warn: unused_results
((), 1); //~ warn: unused_results
```
</td>
<td>
```rust
((), ()); // trivial
((), 1); //~ warn: unused_results
```
</td>
</tr>
<tr>
<td>
tuples are trivial if **any** of their fields are trivial
</td>
<td>
tuples are never trivial
</td>
<td>
tuples are trivial if **all** of their fields are trivial
</td>
</tr>
</table>
Jonathan Brouwer
2fe8b01461