Renamer: improve handling of export children

Ticket: #24027 (closed).

This MR 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.