Improve how we find private method decls. This involved:

- Changed Sema::ObjcActOnStartOfMethodDef() to register the methods with the global pools.
- Changed Sema::ActOnInstanceMessage() to look in global pools (should be much less error prone).
- Added a test case to message.m (for lookup that was broken).

Misc changes while I was investigating this...

- Changed Sema::ActOnAtEnd() to call AddFactoryMethodToGlobalPool (this looked like a cut/paste error).
- Added a comment and tweaked another where I was using the first person.

llvm-svn: 45142