Move the -fms-compatibility using decl check after real access checking

Summary:
This avoids false positives from -Wmicrosoft when name lookup would
normally succeed in standard C++.  This triggered on a common CRTP
pattern in clang, where a derived class would have a private using decl
to pull in members of a dependent base:

class Verifier : InstVisitor<Verifier> {
private:
  using InstVisitor<Verifier>::visit;
  ...
  void anything() {
    visit(); // warned here
  }
};

Real access checks pass here because we're in the context of the
Verifier, but the -Wmicrosoft extension was just looking for the private
access specifier.

Reviewers: rsmith

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2679