node.h: NODE_PRIVATE_RECV

* node.h (NODE_PRIVATE_RECV): name a magic number, `self` as the
  receiver of a setter method call.

* compile.c (private_recv_p), parse.y (attr_receiver): use the
  named macro.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46361 b2dd03c8-39d4-4d8f-98ff-823fe69b080e