The code is actually in ObjectSynchronizer::jni_enter:

if (obj->klass()->is_inline_klass()) {
    ResourceMark rm(THREAD);
    const char* desc = "Cannot synchronize on an instance of value class ";
    const char* className = obj->klass()->external_name();
    size_t msglen = strlen(desc) + strlen(className) + 1;
    char* message = NEW_RESOURCE_ARRAY(char, msglen);
    assert(message != nullptr, "NEW_RESOURCE_ARRAY should have called vm_exit_out_of_memory and not return nullptr");
    THROW_MSG(vmSymbols::java_lang_IdentityException(), className);
  }

The code omits copying the message text to the new array, and just uses the classname as the exception message.