Issue #276 - introduce class BSTR_Argument

src/plugin/ATL_Deprecate.h

 /**
  * \file ATL_Deprecate.h Include package for all the ATL headers, with optional forwarding to detect ATL dependencies.
  *
  * This header is a nexus for refactoring work to remove ATL.
  * The main purpose of this mechanism is to determine the exact scope of the refactoring effort.
  * The lifetime of this header is during the work on #276 Stop using ATL https://issues.adblockplus.org/ticket/276
  *
  * This file implements namespace forwarding that allows detection of ATL symbols.
  * This forwarding is ordinarily disabled, and always disabled in Release configuration for safety.
  * When forwarding is disabled, it simply includes the ATL headers.
@@ skipping 148 matching lines @@
  *   MSDN CString http://msdn.microsoft.com/en-us/library/aa300688%28v=vs.60%29.aspx
  */
 
 #include <string>
 /**
  * Conversion function from ATL:CString to std::wstring
  *
  * The argument cannot be declared 'const' because of the CString API.
  * Reference argument usually does not require explicit temporaries.
  */
-std::wstring to_wstring(ATL::CString& s);
+std::wstring to_wstring(const ATL::CString& s);
 
 /**
  * Conversion function from std::wstring to ATL::CString
  */
 ATL::CString to_CString(const std::wstring& s);