overload_cast MSVC 2015 fix

The current `py::overload_cast` is hitting some ICEs under both MSVC
2015 and clang 3.8 on debian with the rewritten test suites; adding an
empty constexpr constructor to the `overload_cast_impl` class seems to
avoid the ICE.

include/pybind11/common.h

@@ -799,6 +799,8 @@ struct nodelete { template <typename T> void operator()(T*) { } };
 NAMESPACE_BEGIN(detail)
 template <typename... Args>
 struct overload_cast_impl {
+    constexpr overload_cast_impl() {} // MSVC 2015 needs this
+
     template <typename Return>
     constexpr auto operator()(Return (*pf)(Args...)) const noexcept
                               -> decltype(pf) { return pf; }