Use AD.deprecator for IllegalStateError

Previously, ActionDispatch::IllegalStateError was deprecated using
Module#deprecate_constant in 0b4b4c6b96a41ef649f15e1a3df26e28ef95ff24.
This requires the -w flag to be used to actually see the deprecation
warning, and it can not be controlled using ActiveSupport::Deprecator
configuration.

This commit changes the deprecation to use #deprecate_constant from
ActiveSupport::Deprecation::DeprecatedConstantAccessor. This ensures
that the deprecation warning will be printed even without -w, and the
warning can be controlled by configuring ActionDispatch.deprecator

actionpack/lib/action_dispatch.rb

@@ -36,11 +36,14 @@ module Rack
 end
 
 module ActionDispatch
+  include ActiveSupport::Deprecation::DeprecatedConstantAccessor
   extend ActiveSupport::Autoload
 
-  class IllegalStateError < StandardError
+  class DeprecatedIllegalStateError < StandardError
   end
-  deprecate_constant :IllegalStateError
+  deprecate_constant "IllegalStateError", "ActionDispatch::DeprecatedIllegalStateError",
+    message: "ActionDispatch::IllegalStateError is deprecated without replacement.",
+    deprecator: ActionDispatch.deprecator
 
   class MissingController < NameError
   end

actionpack/test/dispatch/response_test.rb

@@ -10,6 +10,12 @@ def setup
     @response.request = ActionDispatch::Request.empty
   end
 
+  def test_illegal_state_error_is_deprecated
+    assert_deprecated(ActionDispatch.deprecator) do
+      ActionDispatch::IllegalStateError
+    end
+  end
+
   def test_can_wait_until_commit
     t = Thread.new {
       @response.await_commit