This doesn't actually fix the encodings for symbols and regex,
unfortunately. But I wanted to get this change in because it is
the last AST change we're going to make before 3.3 is released.

So, if consumers want, they can start to check these flags to
determine the encoding, even though it will be wrong. Then once we
actually set them correctly, everything should work.

https://github.com/ruby/prism/commit/9b35f7e891