Emulate a MSVC bug where if during an using declaration name lookup, the...

Emulate a MSVC bug where if during an using declaration name lookup, the declaration found is unaccessible (private) and that declaration was bring into scope via another using declaration whose target declaration is accessible (public) then no error is generated.

Example:
class A { public: int f();  };
class B : public A { private: using A::f; };
class C : public B { private: using B::f; };

Here, B::f is private so this should fail in Standard C++, but because B::f refers to A::f which is public MSVC accepts it.

This fixes 1 error when parsing MFC code with clang.

llvm-svn: 131896