rnFamInstEqn would mark the name of the type/data
family used in an equation as an occurrence, regardless of what sort
of family it is. Most of the time, this is the correct thing to do.
The exception is closed type families, whose equations constitute its
definition and therefore should not be marked as occurrences.
Overzealously counting the equations of a closed type family as
occurrences can cause certain warnings to not be emitted, as observed
in #18470 (closed).
This fixes #18470 (closed) with a little bit of extra-casing in
rnFamInstEqn. To accomplish this, I added an extra
ClosedTyFamInfo field to the
NonAssocTyFamEqn constructor of
AssocTyFamInfo and refactored the relevant call sites accordingly
so that this information is propagated to