LibJS: Preserve the original this value

As shown in the test added by this patch, it was possible to re-assign
the `this` value of a member function call while it was executing.
Let's copy the original this value like we already do with the callee.

Fixes #2226.

Userland/Libraries/LibJS/Tests/functions/function-assignment-in-arguments.js

@@ -0,0 +1,5 @@
+test("overwriting this during function call still binds the original", () => {
+    let tmp = new Map();
+    // prettier-ignore
+    tmp.set("", tmp = []);
+});