parse.y: allow parenthesed do-block in cmdarg

* parse.y (primary): flush cmdarg flags inside left-paren in a
  command argument, to allow parenthesed do-block as an argument
  without arguments parentheses.  [ruby-core:61950] [Bug #9726]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45637 b2dd03c8-39d4-4d8f-98ff-823fe69b080e