Cleanup in the AppleObjCRuntimeV2 to make descriptors

lighter-weight so that the cache can be populated
faster.

- I Added a ProcessWP to the runtime so I can
  take it out of the individual descriptors, saving
  space;
- I made the constructors for the descriptors
  private so that only the runtime can invoke them; 
  and
- I removed the constructor that takes a ValueObject
  since the logic for using a ValueObject is in the
  runtime.

llvm-svn: 165549