Improved error message about exported type operators.

There is ambiguty between (1) type constructors and (2) data
constructors in export lists, e.g. '%%' can stand for both of them. This
ambiguity is resolved in favor of (2).

If the exported data constructor is not in scope, but type constructor
with the same name is in scope, GHC should suggest adding 'type' keyword
to resolve ambiguity in favor of (1) and enabling 'TypeOperators'
extension.

The patch only extends the error message.

See Trac #11432.

Test Plan: `make test`

Reviewers: simonpj, bgamari, austin

Reviewed By: simonpj

Subscribers: mpickering, thomie, goldfire, kosmikus

Differential Revision: https://phabricator.haskell.org/D1902

GHC Trac Issues: #11432