Prioritise Parent when looking up class sub-binder

When we look up children GlobalRdrElts of a given Parent, we sometimes
would rather prioritise those GlobalRdrElts which have the right Parent,
and sometimes prioritise those that have the right NameSpace:

  - in export lists, we should prioritise NameSpace
  - for class/instance binders, we should prioritise Parent

See Note [childGREPriority] in GHC.Types.Name.Reader.

fixes #23664