Fix ICE with constexpr and friend functions

Fix a crash while parsing this code:

  struct X  {
    friend constexpr int foo(X*) { return 12; }
    static constexpr int j = foo(static_cast<X*>(nullptr));
  };

Differential Revision: http://reviews.llvm.org/D16973

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260675 91177308-0d34-0410-b5e6-96231b3b80d8