Add a comment explaining the test hack

Author: villevoutilainen

gcc/testsuite/g++.dg/contracts/cpp26/throwing-violation-handler.cc

@@ -5,6 +5,10 @@ void handle_contract_violation(const std::contracts::contract_violation&)
   throw 666;
 }
 
+/* ODR hack to the hilt - f is actually noexcept, and has a contract.
+   We do this to avoid the call site knowing that, so that we verify
+   that the function terminates when it calls the throwing handler
+   that we use above. */
 int f(int);
 
 extern bool f_result;