eval.c: rb_frame_callee returns current name

* eval.c (rb_frame_callee): returns the called name of the current
  frame, not the previous frame.
* eval.c (prev_frame_callee, prev_frame_func): rename and make static,
  as these are used by rb_f_method_name() and rb_f_callee_name() only.
* variable.c (set_const_visibility): use the called name.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40921 b2dd03c8-39d4-4d8f-98ff-823fe69b080e