use structured suggestion for "missing mut" label

Fixes #54133.

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

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