Common: Add MoveOnlyFunction.

Source/Core/Common/Functional.h

@@ -0,0 +1,51 @@
+// Copyright 2025 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include <memory>
+
+// TODO C++23: Replace with std::move_only_function.
+
+namespace Common
+{
+
+template <typename T>
+class MoveOnlyFunction;
+
+template <typename R, typename... Args>
+class MoveOnlyFunction<R(Args...)>
+{
+public:
+  using result_type = R;
+
+  MoveOnlyFunction() = default;
+
+  template <typename F>
+  MoveOnlyFunction(F&& f) : m_ptr{std::make_unique<Func<F>>(std::forward<F>(f))}
+  {
+  }
+
+  result_type operator()(Args... args) const { return m_ptr->Invoke(std::forward<Args>(args)...); }
+  explicit operator bool() const { return m_ptr != nullptr; };
+  void swap(MoveOnlyFunction& other) { m_ptr.swap(other.m_ptr); }
+
+private:
+  struct FuncBase
+  {
+    virtual ~FuncBase() = default;
+    virtual result_type Invoke(Args...) = 0;
+  };
+
+  template <typename F>
+  struct Func : FuncBase
+  {
+    Func(F&& f) : func{std::forward<F>(f)} {}
+    result_type Invoke(Args... args) override { return func(std::forward<Args>(args)...); }
+    F func;
+  };
+
+  std::unique_ptr<FuncBase> m_ptr;
+};
+
+}  // namespace Common