Fix smart_holder multiple inheritance tests (#3635)

The original pybind11 holder supported multiple inheritance by recursively creating
type casters until it finds one for the source type, then converting each
value in turn to the next type via typeinfo->implicit_cast

The smart_holder only stored the last implicit_cast, which was incorrect.

This commit changes it to create a list of implicit_cast functions that are
appended to during the recursive type caster creation, and when the time comes
to cast to the destination type, it calls all of them in the correct order.

include/pybind11/detail/smart_holder_type_casters.h

@@ -99,7 +99,9 @@ class modified_type_caster_generic_load_impl {
                 }
                 loaded_v_h = sub_caster.loaded_v_h;
                 loaded_v_h_cpptype = cast.first;
-                implicit_cast = cast.second;
+                // the sub_caster is being discarded, so steal its vector
+                implicit_casts = std::move(sub_caster.implicit_casts);
+                implicit_casts.emplace_back(cast.second);
                 return true;
             }
         }
@@ -180,7 +182,7 @@ class modified_type_caster_generic_load_impl {
             }
             loaded_v_h = foreign_loader->loaded_v_h;
             loaded_v_h_cpptype = foreign_loader->loaded_v_h_cpptype;
-            implicit_cast = foreign_loader->implicit_cast;
+            implicit_casts = foreign_loader->implicit_casts; // SMART_HOLDER_WIP: should this be a copy or move?
             return true;
         }
         return false;
@@ -286,7 +288,7 @@ class modified_type_caster_generic_load_impl {
     void *unowned_void_ptr_from_direct_conversion = nullptr;
     void *unowned_void_ptr_from_void_ptr_capsule = nullptr;
     const std::type_info *loaded_v_h_cpptype = nullptr;
-    void *(*implicit_cast)(void *) = nullptr;
+    std::vector<void *(*)(void *)> implicit_casts;
     value_and_holder loaded_v_h;
     bool reinterpret_cast_deemed_ok = false;
     // Magic number intentionally hard-coded, to guard against class_ holder mixups.
@@ -567,8 +569,10 @@ struct smart_holder_type_caster_load {
 
     T *convert_type(void *void_ptr) const {
         if (void_ptr != nullptr && load_impl.loaded_v_h_cpptype != nullptr
-            && !load_impl.reinterpret_cast_deemed_ok && load_impl.implicit_cast != nullptr) {
-            void_ptr = load_impl.implicit_cast(void_ptr);
+            && !load_impl.reinterpret_cast_deemed_ok && !load_impl.implicit_casts.empty()) {
+            for (auto implicit_cast: load_impl.implicit_casts) {
+                void_ptr = implicit_cast(void_ptr);
+            }
         }
         return static_cast<T *>(void_ptr);
     }