Fix a pretty printer crash on `/***`.

The pretty printer was treating block comments with more than two
asterisks after the first slash (e.g. `/***`) as doc comments (which are
attributes), whereas in actual fact they are just regular comments.
auto merge of #12586 : chris-morgan/rust/fix-pretty-print-slash-star-star-star-crash, r=alexcrichton

The pretty printer was treating block comments with more than two
asterisks after the first slash (e.g. `/***`) as doc comments (which are
attributes), whereas in actual fact they are just regular comments.