parse.y: warn circular argument reference

* parse.y (gettable_gen): warn circular argument reference, for
  transition from 2.1 and earlier.  [ruby-core:65990] [Bug #10314]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48188 b2dd03c8-39d4-4d8f-98ff-823fe69b080e