Fix -Woverloaded-virtual when the using statement refers to a base declaration of...

Fix -Woverloaded-virtual when the using statement refers to a base declaration of a virtual function.

GCC and Clang both do not warn on:

  struct a { virtual void func(); };
  struct b: a { virtual void func(); void func(int); };
  struct c: b { void func(int); using b::func; };

but if the "using" was using a::func GCC would still remain silent where Clang
would warn. This change makes Clang consistent with GCC's existing behavior.

llvm-svn: 166154