Implement disambiguation of base class members via a

nested-name-specifier. For example, this allows member access in
diamond-shaped hierarchies like:

  struct Base {
    void Foo();
    int Member;
  };

  struct D1 : public Base {};
  struct D2 : public Base {};

  struct Derived : public D1, public D2 { }

  void Test(Derived d) {
    d.Member = 17; // error: ambiguous cast from Derived to Base
    d.D1::Member = 17; // error: okay, modify D1's Base's Member
  }

Fixes PR5820 and <rdar://problem/7535045>. Also, eliminate some
redundancy between Sema::PerformObjectMemberConversion() and
Sema::PerformObjectArgumentInitialization() -- the latter now calls
the former.

llvm-svn: 97674