Renamer: improve handling of export children

This commit refactors the 'childGREPriority' function which is used when
renaming subordinate items in export lists and class declarations.

Instead of having a complicated LookupChild parameter, we now simply pass the
GREInfo of the parent, which allows us to decide what is a valid child:

  - classes can have children that are in the type constructor namespace,
  - promoted data constructors should be treated the same as normal data
    constructors.

Fixes #24027