Ignore dead code in function-epilogue demo

    warning: struct `S` is never constructed
     --> function-epilogue/demo/main.rs:3:12
      |
    3 | pub struct S(i32);
      |            ^
      |
      = note: `#[warn(dead_code)]` on by default

function-epilogue/demo/main.rs

@@ -35,4 +35,6 @@ impl S {
     }
 }
 
-fn main() {}
+fn main() {
+    let _ = S;
+}