Look through using decls when classifying implicit member access

Clang will now accept this valid C++11 code:
  struct A { int field; };
  struct B : A {
    using A::field;
    enum { TheSize = sizeof(field) };
  };

Previously we would classify the 'field' reference as something other
than a field, and then forget to apply the C++11 rule to allow
non-static data member references in unevaluated contexts.

This usually arises in class templates that want to reference fields of
a dependent base in an unevaluated context outside of an instance
method. Such contexts do not allow references to 'this', so the only way
to access the field is with a using decl and an implicit member
reference.

llvm-svn: 250839