Add comment to test

Author: BoxyUwU

tests/ui/const-generics/issues/cg-in-dyn-issue-128176.rs

@@ -1,6 +1,7 @@
 //@ check-pass
 
-// Regression test for #128176.
+// Regression test for #128176. Previously we would call `type_of` on the `1` anon const
+// before the anon const had been lowered and had the `type_of` fed with a result.
 
 #![feature(generic_const_exprs)]
 #![feature(dyn_compatible_for_dispatch)]