variable.c: fix private constant message

* variable.c (rb_const_search): raise with the actual class/module
  name which defines the private constant.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56117 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_module.rb

@@ -1360,6 +1360,9 @@ class TestModule < Test::Unit::TestCase
     c.const_set(:FOO, "foo")
     $VERBOSE = verbose
     assert_raise(NameError) { c::FOO }
+    assert_raise_with_message(NameError, /#{c}::FOO/) do
+      Class.new(c)::FOO
+    end
   end
 
   def test_private_constant2