not raise an exception even when the constant is private.  Instead,
  rb_public_const_get_* and rb_public_const_defined_* are introduced,
  which raise an exception when the referring constant is private.
  see [ruby-core:32912].

* vm_insnhelper.c (vm_get_ev_const): use rb_public_const_get_* instead
  of rb_const_get_* to follow the constant visibility when user code
  refers a constant.

* test/ruby/test_marshal.rb (test_marshal_private_class): add a test.
  This test had failed because of incompatibility of rb_const_get.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@30713 b2dd03c8-39d4-4d8f-98ff-823fe69b080e