don't emit unused_results lint for tuples of "trivial" types

[WaffleLapkin]

r? @jdonszelmann
Fixes #153144.

So it turns out #153018 had a sneaky behavior change in the way tuples are handled and the old behavior wasn't tested.

Consider these tuples:

((), ());
((), 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:

stable	After #153018	After this PR
((), ()); // trivial ((), 1); // trivial	((), ()); //~ warn: unused_results ((), 1); //~ warn: unused_results	((), ()); // trivial ((), 1); //~ warn: unused_results
tuples are trivial if any of their fields are trivial	tuples are never trivial	tuples are trivial if all of their fields are trivial