isDataTyCon should be False for all type families, even data type families
isDataTyCon advertises that it's true of "data types that are definitely represented by heap-allocated constructors. These are srcutinised by Core-level @case@ expressions, and they get info tables allocated for them." Type-family TyCons never have this property, not even data type families. It's the *instance* TyCons that do. I hope that this change does not break anything that somehow relied on the old (wrong) semantics.