Fix #15941 by only special-casing visible infix applications

Summary:
The iface pretty-printer had a special case for an
application of an infix type constructor to two arguments. But this
didn't take the visibilities of the arguments into account, which
could lead to strange output like `@{LiftedRep} -> @{LiftedRep}` when
`-fprint-explicit-kinds` was enabled (#15941). The fix is relatively
straightforward: simply plumb through the visibilities of each
argument, and only trigger the special case for infix applications
if both arguments are visible (i.e., required).

Test Plan: make test TEST=T15941

Reviewers: goldfire, bgamari, monoidal

Reviewed By: goldfire, monoidal

Subscribers: simonpj, rwbarton, carter

GHC Trac Issues: #15941

Differential Revision: https://phabricator.haskell.org/D5375

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