LibJS: Use CreateUnmappedArgumentsObject for non-simple parameter lists

This patch implements the IsSimpleParameterList static semantics for
ordinary function objects.

We now also create an unmapped arguments object for callee contexts
with non-simple parameter lists, instead of only doing it in strict
mode. Covered by test262.

Userland/Libraries/LibJS/Runtime/FunctionObject.h

@@ -57,12 +57,17 @@ public:
     ThisMode this_mode() const { return m_this_mode; }
     void set_this_mode(ThisMode this_mode) { m_this_mode = this_mode; }
 
+    // This is for IsSimpleParameterList (static semantics)
+    bool has_simple_parameter_list() const { return m_has_simple_parameter_list; }
+
 protected:
     virtual void visit_edges(Visitor&) override;
 
     explicit FunctionObject(Object& prototype);
     FunctionObject(Value bound_this, Vector<Value> bound_arguments, Object& prototype);
 
+    void set_has_simple_parameter_list(bool b) { m_has_simple_parameter_list = b; }
+
 private:
     virtual bool is_function() const override { return true; }
     Value m_bound_this;
@@ -70,6 +75,7 @@ private:
     Value m_home_object;
     ConstructorKind m_constructor_kind = ConstructorKind::Base;
     ThisMode m_this_mode { ThisMode::Global };
+    bool m_has_simple_parameter_list { false };
 };
 
 }

Userland/Libraries/LibJS/Runtime/OrdinaryFunctionObject.cpp

@@ -68,6 +68,17 @@ OrdinaryFunctionObject::OrdinaryFunctionObject(GlobalObject& global_object, cons
         set_this_mode(ThisMode::Strict);
     else
         set_this_mode(ThisMode::Global);
+
+    // 15.1.3 Static Semantics: IsSimpleParameterList, https://tc39.es/ecma262/#sec-static-semantics-issimpleparameterlist
+    set_has_simple_parameter_list(all_of(m_parameters.begin(), m_parameters.end(), [&](auto& parameter) {
+        if (parameter.is_rest)
+            return false;
+        if (parameter.default_value)
+            return false;
+        if (!parameter.binding.template has<FlyString>())
+            return false;
+        return true;
+    }));
 }
 
 void OrdinaryFunctionObject::initialize(GlobalObject& global_object)

Userland/Libraries/LibJS/Runtime/VM.cpp

@@ -369,7 +369,7 @@ Value VM::get_variable(const FlyString& name, GlobalObject& global_object)
             if (possible_match.has_value())
                 return possible_match.value().value;
             if (!context.arguments_object) {
-                if (context.function->is_strict_mode()) {
+                if (context.function->is_strict_mode() || !context.function->has_simple_parameter_list()) {
                     context.arguments_object = create_unmapped_arguments_object(global_object, context.arguments);
                 } else {
                     // FIXME: This code path is completely ad-hoc.