Fix #14390 by making toIfaceTyCon aware of equality

GHC was panicking when pretty-printing a heterogeneous
equality type constructor (#14390) because the function which
produced the type constructor, `toIfaceTyCon`, wasn't attaching the
appropriate `IfaceTyConSort` for equality type constructors, which
is `IfaceEqualityTyCon`. This is fixed easily enough.

Test Plan: make test TEST=T14390

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #14390

Differential Revision: https://phabricator.haskell.org/D4132