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

compiled/filter/WhitelistFilter.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
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
  */
 
 #pragma once
 
 #include "RegExpFilter.h"
 
 class WhitelistFilter : public RegExpFilter
 {
 public:
+  static constexpr Type classType = Type::WHITELIST;
   explicit WhitelistFilter(const String& text, const RegExpFilterData& data);
 };
-
-template<>
-inline WhitelistFilter* Filter::As<WhitelistFilter>()
-{
-  if ((mType & Type::WHITELIST) == Type::WHITELIST)
-    return nullptr;
-
-  return static_cast<WhitelistFilter*>(this);
-}