Common: Add variant utilities

Source/Core/Common/Common.vcxproj

@@ -159,6 +159,7 @@
     <ClInclude Include="TraversalClient.h" />
     <ClInclude Include="TraversalProto.h" />
     <ClInclude Include="UPnP.h" />
+    <ClInclude Include="VariantUtil.h" />
     <ClInclude Include="Version.h" />
     <ClInclude Include="WorkQueueThread.h" />
     <ClInclude Include="x64ABI.h" />

Source/Core/Common/VariantUtil.h

@@ -0,0 +1,26 @@
+// Copyright 2018 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#include <variant>
+
+namespace detail
+{
+template <typename... From>
+struct VariantCastProxy
+{
+  const std::variant<From...>& v;
+
+  template <typename... To>
+  operator std::variant<To...>() const
+  {
+    return std::visit([](auto&& arg) { return std::variant<To...>{arg}; }, v);
+  }
+};
+}  // namespace detail
+
+template <typename... From>
+auto VariantCast(const std::variant<From...>& v)
+{
+  return detail::VariantCastProxy<From...>{v};
+}