AK: Return early from swap() when swapping the same object

When swapping the same object, we could end up with a double-free error.
This was found while quick-sorting a Vector of Variants holding complex
types, reproduced by the new swap_same_complex_object test case.

Tests/AK/TestStdLibExtras.cpp

@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2021, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibTest/TestSuite.h>
+
+#include <AK/Optional.h>
+#include <AK/StdLibExtras.h>
+#include <AK/StringView.h>
+#include <AK/Variant.h>
+#include <AK/Vector.h>
+
+TEST_CASE(swap)
+{
+    int i = 4;
+    int j = 6;
+
+    swap(i, j);
+
+    EXPECT_EQ(i, 6);
+    EXPECT_EQ(j, 4);
+}
+
+TEST_CASE(swap_same_value)
+{
+
+    int i = 4;
+    swap(i, i);
+    EXPECT_EQ(i, 4);
+}
+
+TEST_CASE(swap_same_complex_object)
+{
+    struct Type1 {
+        StringView foo;
+    };
+    struct Type2 {
+        Optional<Type1> foo;
+        Vector<Type1> bar;
+    };
+
+    Variant<Type1, Type2> value1 { Type1 { "hello"sv } };
+    Variant<Type1, Type2> value2 { Type2 { {}, { { "goodbye"sv } } } };
+
+    swap(value1, value2);
+
+    EXPECT(value1.has<Type2>());
+    EXPECT(value2.has<Type1>());
+
+    swap(value1, value1);
+
+    EXPECT(value1.has<Type2>());
+    EXPECT(value2.has<Type1>());
+}