Method swap

internal/concurrentqueue_internal_debug.h:60–60

		DebugFreeList() : head(nullptr) { }
		DebugFreeList(DebugFreeList&& other) : head(other.head) { other.head = nullptr; }
		void swap(DebugFreeList& other) { std::swap(head, other.head); }
		
		inline void add(N* node)
		{