Allow refining a frozen class

Doing so modifies the class's method table, but not in a way that should
be detectable from Ruby, so it may be safe to avoid checking if the
class is frozen.

Fixes [Bug #11669]

test/ruby/test_refinement.rb

@@ -2405,6 +2405,39 @@ class TestRefinement < Test::Unit::TestCase
     end
   end
 
+  def test_refine_frozen_class
+    singleton_class.instance_variable_set(:@x, self)
+    class << self
+      c = Class.new do
+        def foo
+          :cfoo
+        end
+      end
+      foo = Module.new do
+        refine c do
+          def foo
+            :rfoo
+          end
+        end
+      end
+      using foo
+      @x.assert_equal(:rfoo, c.new.foo)
+      c.freeze
+      foo.module_eval do
+        refine c do
+          def foo
+            :rfoo2
+          end
+          def bar
+            :rbar
+          end
+        end
+      end
+      @x.assert_equal(:rfoo2, c.new.foo)
+      @x.assert_equal(:rbar, c.new.bar, '[ruby-core:71391] [Bug #11669]')
+    end
+  end
+
   private
 
   def eval_using(mod, s)

vm_method.c

@@ -714,7 +714,9 @@ rb_method_entry_make(VALUE klass, ID mid, VALUE defined_class, rb_method_visibil
 	}
     }
 
-    rb_class_modify_check(klass);
+    if (type != VM_METHOD_TYPE_REFINED) {
+       rb_class_modify_check(klass);
+    }
 
     if (FL_TEST(klass, RMODULE_IS_REFINEMENT)) {
 	VALUE refined_class = rb_refinement_module_get_refined_class(klass);