Add another case of fallback to () avoid breakage

This adds src/test/ui/never_type/fallback-closure-ret.rs as a test case which
showcases the failure mode fixed by this commit.

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -15,6 +15,7 @@ mod object_safety;
 mod on_unimplemented;
 mod project;
 pub mod query;
+pub(crate) mod relationships;
 mod select;
 mod specialize;
 mod structural_match;