Use shimmed std::as_const instead of qAsConst

include/stdshims.h

@@ -5,11 +5,12 @@
 #define STDSHIMS_H
 
 #include <memory>
+#include <type_traits>
 #include <utility>
 
 #if (__cplusplus >= 201402L || _MSC_VER)
 #ifndef _MSC_VER
-#warning "This file should now be removed! The functions it provides are part of the C++14 standard."
+#warning "This part of this file should now be removed! The functions it provides are part of the C++14 standard."
 #endif
 using std::make_unique;
 
@@ -30,5 +31,24 @@ std::unique_ptr<T, Deleter> make_unique(Args&&... args)
 }
 #endif
 
+#if (__cplusplus >= 201703L || _MSC_VER >= 1914)
+#ifndef _MSC_VER
+#warning "This part of this file should now be removed! The functions it provides are part of the C++17 standard."
+#endif
+using std::as_const;
+
+#else
+
+/// Shim for http://en.cppreference.com/w/cpp/utility/as_const
+template <typename T>
+constexpr typename std::add_const<T>::type& as_const(T& t) noexcept
+{
+    return t;
+}
+
+template <typename T>
+void as_const(const T&&) = delete;
+#endif
+
 #endif // include guard