5 cleanups to ObjCObjectPointerType work:

- Remove Sema::CheckPointeeTypesForAssignment(), a temporary API I added to ease migration to ObjCObjectPointerType. Convert Sema::CheckAssignmentConstraints() to no longer depend on the temporary API.
- Sema::ConvertDeclSpecToType(): Replace a couple FIXME's with an important comment/example.
- Sema::GetTypeForDeclarator(): Get the protocol's from the interface, NOT the declspec (to support the following C typedef idiom: "typedef C<P> T; T *obj").
- Sema::ObjCQualifiedIdTypesAreCompatible(): Removed some dead code.
- ASTContext::getObjCEncodingForTypeImpl(): Some minor cleanups.

llvm-svn: 76443