parse.y: turn dynamically interned Symbol into an ID

* parse.y (rb_id_attrset): turn dynamically interned Symbol into
  an ID, since rb_str_dynamic_intern returns a Symbol but not an
  ID.  [ruby-core:62226] [Bug #9787]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45782 b2dd03c8-39d4-4d8f-98ff-823fe69b080e