Reimplement (de-)serialization of Objective-C categories to eliminate

the direct serialization of the linked-list structure. Instead, use a
scheme similar to how we handle redeclarations, with redeclaration
lists on the side. This addresses several issues:
  - In cases involving mixing and matching of many categories across
  many modules, the linked-list structure would not be consistent
  across different modules, and categories would get lost.
  - If a module is loaded after the class definition and its other
  categories have already been loaded, we wouldn't see any categories
  in the newly-loaded module.

llvm-svn: 149112