Tweak to the pretty type parser to recognize that `->` is a special token.

Tweak to the pretty type parser to recognize that `->` is a special token that
shouldn't be split into two characters.  This change allows dialect
types to wrap function types as in `!my.ptr_type<(i32) -> i32>`.

Closes tensorflow/mlir#105

COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/105 from schweitzpgi:parse-arrow 8b2d768053f419daae5a1a864121a44c4319acbe
PiperOrigin-RevId: 265986240