Issue 5258 - Implement Filter::As<>() method to make working with filters easier from C++ code

compiled/filter/InvalidFilter.h

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 13 matching lines @@
 {
 public:
   explicit InvalidFilter(const String& text, const String& reason);
   const String& BINDINGS_EXPORTED GetReason() const
   {
     return mReason;
   };
 private:
   OwnedString mReason;
 };
+
+template<>
+inline InvalidFilter* Filter::As<InvalidFilter>()
+{
+  if ((mType & Type::INVALID) == Type::INVALID)
+    return nullptr;
+
+  return static_cast<InvalidFilter*>(this);
+}