Don't trigger unused_result on functions returning empty enums

src/test/ui/issue-43806.rs

@@ -12,15 +12,22 @@
 
 #![deny(unused_results)]
 
+enum Void {}
+
 fn foo() {}
 
 fn bar() -> ! {
     loop {}
 }
 
+fn baz() -> Void {
+    loop {}
+}
+
 fn qux() {
     foo();
     bar();
+    baz();
 }
 
 fn main() {}