The opt_sig production is defined as

  opt_sig :: { ([AddAnn],Maybe (LHsType RdrName)) }
          : {- empty -}                   { ([],Nothing) }
          | '::' sigtype                  { ([mj AnnDcolon $1],Just $2) }

It is used in the alt and decl_no_th productions, but neither of them
add the returned annotations.

This commit captures the annotations in the calling productions.

Reviewed By: austin

Differential Revision: https://phabricator.haskell.org/D822

GHC Trac Issues: #10254