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_middle/src/ty/mod.rs

@@ -2090,3 +2090,16 @@ impl<'tcx> fmt::Debug for SymbolName<'tcx> {
         fmt::Display::fmt(&self.name, fmt)
     }
 }
+
+#[derive(Debug, Default, Copy, Clone)]
+pub struct FoundRelationships {
+    /// This is true if we identified that this Ty (`?T`) is found in a `?T: Foo`
+    /// obligation, where:
+    ///
+    ///  * `Foo` is not `Sized`
+    ///  * `(): Foo` may be satisfied
+    pub self_in_trait: bool,
+    /// This is true if we identified that this Ty (`?T`) is found in a `<_ as
+    /// _>::AssocType = ?T`
+    pub output: bool,
+}