* defs/keywords (alias, undef): symbol literals are allowed.
* parse.y (parse_percent): should parse symbol literals for alias
  and undef.  [ruby-dev:47681] [Bug #8851]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54131 b2dd03c8-39d4-4d8f-98ff-823fe69b080e