parse.y: non-local/const attrset

* parse.y (rb_enc_symname_type): allow ID_ATTRSET for ID_INSTANCE,
  ID_GLOBAL, ID_CLASS, ID_JUNK too.  [Bug #8756]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42509 b2dd03c8-39d4-4d8f-98ff-823fe69b080e