* insns.def (defineclass): private constants should not be accessed
  by scoped module definitions.  The bug was introduced in r38495.

* test/ruby/test_module.rb: related test.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38772 b2dd03c8-39d4-4d8f-98ff-823fe69b080e