Fix unused results from mem::replace

src/test/ui/imports/import-in-block.rs

@@ -4,7 +4,7 @@
 pub fn main() {
     use std::mem::replace;
     let mut x = 5;
-    replace(&mut x, 6);
+    let _ = replace(&mut x, 6);
     {
         use std::mem::*;
         let mut y = 6;

src/test/ui/issues/issue-23611-enum-swap-in-drop.rs

@@ -154,7 +154,7 @@ impl<'a> Drop for E<'a> {
         };
 
         if do_drop {
-            mem::replace(self, E::A(GaspA(f_a, 0xA3A0, log, D::new("drop", 6, log)), true));
+            let _ = mem::replace(self, E::A(GaspA(f_a, 0xA3A0, log, D::new("drop", 6, log)), true));
         }
     }
 }