Ignore warnings from polymorphization tests

Author: saethlin

tests/ui/polymorphization/issue-74614.rs

@@ -1,5 +1,6 @@
 //@ compile-flags:-Zpolymorphize=on
 //@ build-pass
+//@ dont-check-compiler-stderr
 
 fn test<T>() {
  std::mem::size_of::<T>();

tests/ui/polymorphization/issue-74614.stderr

@@ -1,5 +1,6 @@
 //@ compile-flags:-Zpolymorphize=on
 //@ build-pass
+//@ dont-check-compiler-stderr
 
 use std::any::TypeId;