Common: Add a std::optional implementation

std::optional makes a few things a bit neater and less error prone.
However, we still cannot use C++17 (unfortunately), so this commit
adds an implementation of std::optional that we can use right now.

Based on https://github.com/tensorflow/tensorflow/blob/master/tensorflow/core/lib/gtl/optional.h
which seems to be fairly similar to C++17's <optional> and standards
compliant. It's one of the few implementations that handle propagating
type traits like copy constructibility, just like libc++/libstdc++.

Source/Core/Common/Compat/in_place.h

@@ -0,0 +1,43 @@
+// Copyright 2017 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+// MPark.Variant
+//
+// Copyright Michael Park, 2015-2017
+//
+// Distributed under the Boost Software License, Version 1.0.
+// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
+
+#include <cstddef>
+
+namespace std
+{
+struct in_place_t
+{
+  explicit in_place_t() = default;
+};
+
+template <std::size_t I>
+struct in_place_index_t
+{
+  explicit in_place_index_t() = default;
+};
+
+template <typename T>
+struct in_place_type_t
+{
+  explicit in_place_type_t() = default;
+};
+
+constexpr in_place_t in_place{};
+
+template <std::size_t I>
+constexpr in_place_index_t<I> in_place_index{};
+
+template <typename T>
+constexpr in_place_type_t<T> in_place_type{};
+
+}  // namespace std