Setting record.default_holder correctly for PYBIND11_USE_SMART_HOLDER_AS_DEFAULT.

With this test_class.cpp builds and even mostly runs, except
`test_multiple_instances_with_same_pointer`, which segfaults because it is
using a `unique_ptr` holder but `smart_holder` `type_caster`.

Also adding `static_assert`s to generate build errors for such situations,
but guarding with `#if 0` to first pivot to test_factory_constructors.cpp.

include/pybind11/pybind11.h

@@ -1294,7 +1294,16 @@ public:
         record.holder_size = sizeof(holder_type);
         record.init_instance = init_instance;
         record.dealloc = dealloc;
+#ifndef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
         record.default_holder = detail::is_instantiation<std::unique_ptr, holder_type>::value;
+#else
+        record.default_holder = std::is_same<holder_type, smart_holder>::value;
+#if 0
+        static_assert(!(detail::is_instantiation<std::unique_ptr, holder_type>::value && detail::is_smart_holder_type_caster<type>::value));
+        static_assert(!(detail::is_instantiation<std::shared_ptr, holder_type>::value && detail::is_smart_holder_type_caster<type>::value));
+        static_assert(detail::is_smart_holder_type_caster<type>::value == std::is_same<holder_type, smart_holder>::value);
+#endif
+#endif
 
         set_operator_new<type>(&record);