parse.y: use rb_fstring_new
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46690 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
parse.y: use rb_fstring_new
with a new string instance.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46690 b2dd03c8-39d4-4d8f-98ff-823fe69b080e