LibJS: Add an optimization to avoid needless arguments object creation

This gives FunctionNode a "might need arguments object" boolean flag and
sets it based on the simplest possible heuristic for this: if we
encounter an identifier called "arguments" or "eval" up to the next
(nested) function declaration or expression, we won't need an arguments
object. Otherwise, we *might* need one - the final decision is made in
the FunctionDeclarationInstantiation AO.

Now, this is obviously not perfect. Even if you avoid eval, something
like `foo.arguments` will still trigger a false positive - but it's a
start and already massively cuts down on needlessly allocated objects,
especially in real-world code that is often minified, and so a full
"arguments" identifier will be an actual arguments object more often
than not.

To illustrate the actual impact of this change, here's the number of
allocated arguments objects during a full test-js run:

Before:
- Unmapped arguments objects: 78765
- Mapped arguments objects: 2455

After:
- Unmapped arguments objects: 18
- Mapped arguments objects: 37

This results in a ~5% speedup of test-js on my Linux host machine, and
about 3.5% on i686 Serenity in QEMU (warm runs, average of 5).

The following microbenchmark (calling an empty function 1M times) runs
25% faster on Linux and 45% on Serenity:

    function foo() {}
    for (var i = 0; i < 1_000_000; ++i)
        foo();

test262 reports no changes in either direction, apart from a speedup :^)

Userland/Libraries/LibJS/Runtime/ECMAScriptFunctionObject.h

@@ -28,9 +28,9 @@ public:
         Global,
     };
 
-    static ECMAScriptFunctionObject* create(GlobalObject&, FlyString name, Statement const& ecmascript_code, Vector<FunctionNode::Parameter> parameters, i32 m_function_length, Environment* parent_scope, FunctionKind, bool is_strict, bool is_arrow_function = false);
+    static ECMAScriptFunctionObject* create(GlobalObject&, FlyString name, Statement const& ecmascript_code, Vector<FunctionNode::Parameter> parameters, i32 m_function_length, Environment* parent_scope, FunctionKind, bool is_strict, bool might_need_arguments_object = true, bool is_arrow_function = false);
 
-    ECMAScriptFunctionObject(FlyString name, Statement const& ecmascript_code, Vector<FunctionNode::Parameter> parameters, i32 m_function_length, Environment* parent_scope, Object& prototype, FunctionKind, bool is_strict, bool is_arrow_function = false);
+    ECMAScriptFunctionObject(FlyString name, Statement const& ecmascript_code, Vector<FunctionNode::Parameter> parameters, i32 m_function_length, Environment* parent_scope, Object& prototype, FunctionKind, bool is_strict, bool might_need_arguments_object, bool is_arrow_function);
     virtual void initialize(GlobalObject&) override;
     virtual ~ECMAScriptFunctionObject();
 
@@ -100,6 +100,7 @@ private:
     Optional<Bytecode::Executable> m_bytecode_executable;
     i32 m_function_length { 0 };
     FunctionKind m_kind { FunctionKind::Regular };
+    bool m_might_need_arguments_object { true };
     bool m_is_arrow_function { false };
     bool m_has_simple_parameter_list { false };
 };