use structured suggestion for "missing mut" label

Fixes #54133.

src/test/ui/liveness/liveness-assign/liveness-assign-imm-local-with-drop.rs

@@ -14,7 +14,8 @@
 fn test() {
     let b = Box::new(1); //[ast]~ NOTE first assignment
                          //[mir]~^ NOTE first assignment
-                         //[mir]~| NOTE consider changing this to `mut b`
+                         //[mir]~| HELP make this binding mutable
+                         //[mir]~| SUGGESTION mut b
     drop(b);
     b = Box::new(2); //[ast]~ ERROR cannot assign twice to immutable variable
                      //[mir]~^ ERROR cannot assign twice to immutable variable `b`