[ObjC][Sema] Avoid warning about a call to an instance method on an

instance of a qualified Class object when that instance method comes from
a protocol that's implemented by NSObject

Instance methods from a root class like NSObject are also class methods because
the metaclass of root class derives from that root class. Therefore, we can
avoid the warning for instances of qualified Class objects that point to classes
that derive from NSObject. Note that we actually don't know if a Class instance
points to a class that derives from NSObject at compile-time, so we have to
make a reasonable assumption that the majority of instances will do so.

rdar://22812517

llvm-svn: 297862