parse.y: symbol names must be ascii-compatible

* parse.y (rb_enc_symname_type): encoding of symbol names must be
  ascii-compatible, reject ascii-incompatible encodings.

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44836 b2dd03c8-39d4-4d8f-98ff-823fe69b080e