[ObjC++] Conversions from specialized to non-specialized Objective-C generic

object types should be preferred over conversions to other object pointers

This change ensures that Clang will select the correct overload for the
following code sample:

  void overload(Base *b);
  void overload(Derived *d);
  void test(Base<Base *> b) {
    overload(b); // Select overload(Base *), not overload(Derived *)
  }

rdar://20124827

Differential Revision: https://reviews.llvm.org/D31597

llvm-svn: 299648