Respect through association scopes when used with polymorphic

When the `source_type` option is passed to a has_many through, the
resulting `Reflection` will be an instance of `PolymorphicReflection`
and not `ThroughReflection`, meaning that it will ignore the scopes that
it needs to apply from the through reflections. This adds an additional
delegation call to remedy this. I've been finding the reflection code
completely impenetrable lately, it could use some major love.

Fixes #22726

activerecord/lib/active_record/reflection.rb

@@ -995,7 +995,7 @@ def type
       end
 
       def constraints
-        [source_type_info]
+        @reflection.constraints + [source_type_info]
       end
 
       def source_type_info

activerecord/test/cases/associations/join_model_test.rb

@@ -363,6 +363,13 @@ def test_has_many_polymorphic_with_source_type
     assert_equal posts(:welcome, :thinking).sort_by(&:id), tags(:general).tagged_posts.sort_by(&:id)
   end
 
+  def test_has_many_polymorphic_associations_merges_through_scope
+    Tag.has_many :null_taggings, -> { none }, class_name: :Tagging
+    Tag.has_many :null_tagged_posts, :through => :null_taggings, :source => 'taggable', :source_type => 'Post'
+    assert_equal [], tags(:general).null_tagged_posts
+    refute_equal [], tags(:general).tagged_posts
+  end
+
   def test_eager_has_many_polymorphic_with_source_type
     tag_with_include = Tag.all.merge!(:includes => :tagged_posts).find(tags(:general).id)
     desired = posts(:welcome, :thinking)