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

compiled/filter/RegExpFilter.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 93 matching lines @@
   SitekeySet* GetSitekeys() const override;
 public:
   explicit RegExpFilter(Type type, const String& text, const RegExpFilterData& data);
   ~RegExpFilter();
   static Type Parse(DependentString& text, DependentString& error,
       RegExpFilterData& data);
   static void GenerateCustomBindings();
   bool BINDINGS_EXPORTED Matches(const String& location, int typeMask,
       DependentString& docDomain, bool thirdParty, const String& sitekey) const;
 };
+
+template<>
+inline RegExpFilter* Filter::As<RegExpFilter>()
+{
+  if ((mType & Type::REGEXP) == Type::REGEXP)
+    return nullptr;
+
+  return static_cast<RegExpFilter*>(this);
+}