vm_eval.c: exclude hidden variables

* vm_eval.c (rb_f_local_variables): exclude variables hidden by
  shadowing.  [ruby-core:60501] [Bug #9486]
* vm.c (collect_local_variables_in_iseq): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45850 b2dd03c8-39d4-4d8f-98ff-823fe69b080e