Before this patch, we maintained two different grammars for types, one that
accepted Haddock comments and one that did not:

  -- No documentation
  type
    : btype            { ... }
    | btype '->' ctype { ... }

  -- Documentation
  typedoc
    : btype                       { ... }
    | btype docprev               { ... }
    | docnext btype               { ... }
    | btype '->'     ctypedoc     { ... }
    | btype docprev '->' ctypedoc { ... }
    | docnext btype '->' ctypedoc { ... }

See the (now deleted)  Note [ctype and ctypedoc]
    and (also deleted) Note [Constr variatons of non-terminals]

This led to a fair share of duplication, with many rules having
a parallel no-documentation rule:

     Documentation  | No documentation
    ----------------+------------------
     typedoc        | type
     ctypedoc       | ctype
     ktypedoc       | ktype
     sigtypedoc     | sigtype
     constr_ftype   | ftype
     constr_btype   | btype
     constr_context | context

With this patch, we always parse the comments, and then either
reject or accept them in a later validation step.