Before this refactoring:

* DerivInfo for data family instances was returned from tcTyAndClassDecls
* DerivInfo for data declarations was generated with mkDerivInfos and added at a
  later stage of the pipeline in tcInstDeclsDeriv

After this refactoring:

* DerivInfo for both data family instances and data declarations is returned from
  tcTyAndClassDecls in a single list.

This uniform treatment results in a more convenient arrangement to fix #16731.