Eliminate Sema::ObjCImplementations, relying instead on name lookup. What's...

Eliminate Sema::ObjCImplementations, relying instead on name lookup. What's good for uniformity is good for PCH (or is it the other way around?). 

As part of this, make ObjCImplDecl inherit from NamedDecl (since
ObjCImplementationDecls now need to have names so that they can be
found). This brings ObjCImplDecl very, very close to
ObjCContainerDecl; we may be able to merge them soon.

llvm-svn: 69941