LibJS: Return non-object argument unaltered from Object.setPrototypeOf()

This was missing step 3 from the spec:

    3. If Type(O) is not Object, return O.

Also use RequireObjectCoercible() for a better error message and make
the rest of the code a bit easier to read and more similar to the spec
text.

Userland/Libraries/LibJS/Tests/builtins/Object/Object.setPrototypeOf.js

@@ -9,6 +9,13 @@ describe("correct behavior", () => {
         expect(Object.setPrototypeOf(o, p)).toBe(o);
         expect(Object.getPrototypeOf(o)).toBe(p);
     });
+
+    test("non-object argument is returned without being coerced to object", () => {
+        let o = 42;
+        let p = {};
+        expect(Object.setPrototypeOf(o, p)).toBe(o);
+        expect(Object.getPrototypeOf(o)).toBe(Number.prototype);
+    });
 });
 
 describe("errors", () => {