Big 853: replace enable_if in Ref<> ctor by static assertions and add failtests for Ref<>

failtest/ref_1.cpp

@@ -0,0 +1,18 @@
+#include "../Eigen/Core"
+
+#ifdef EIGEN_SHOULD_FAIL_TO_BUILD
+#define CV_QUALIFIER const
+#else
+#define CV_QUALIFIER
+#endif
+
+using namespace Eigen;
+
+void call_ref(Ref<VectorXf> a) { }
+
+int main()
+{
+  VectorXf a(10);
+  CV_QUALIFIER VectorXf& ac(a);
+  call_ref(ac);
+}