LibJS: Mark two JS::Reference functions `const`

These two are inherently const, and the next commit needs to call them
on a const object, so let's just mark them const.

Userland/Libraries/LibJS/Runtime/Reference.cpp

@@ -73,7 +73,7 @@ void Reference::put_value(GlobalObject& global_object, Value value)
     }
 }
 
-void Reference::throw_reference_error(GlobalObject& global_object)
+void Reference::throw_reference_error(GlobalObject& global_object) const
 {
     auto& vm = global_object.vm();
     if (!m_name.is_valid())
@@ -83,7 +83,7 @@ void Reference::throw_reference_error(GlobalObject& global_object)
 }
 
 // 6.2.4.5 GetValue ( V ), https://tc39.es/ecma262/#sec-getvalue
-Value Reference::get_value(GlobalObject& global_object, bool throw_if_undefined)
+Value Reference::get_value(GlobalObject& global_object, bool throw_if_undefined) const
 {
     if (is_unresolvable()) {
         throw_reference_error(global_object);

Userland/Libraries/LibJS/Runtime/Reference.h

@@ -98,13 +98,13 @@ public:
     }
 
     void put_value(GlobalObject&, Value);
-    Value get_value(GlobalObject&, bool throw_if_undefined = true);
+    Value get_value(GlobalObject&, bool throw_if_undefined = true) const;
     bool delete_(GlobalObject&);
 
     String to_string() const;
 
 private:
-    void throw_reference_error(GlobalObject&);
+    void throw_reference_error(GlobalObject&) const;
 
     BaseType m_base_type { BaseType::Unresolvable };
     union {