Xcode: Factor out XCODE_ATTRIBUTE_ variant filter (#14947)

Move the variant=<config> filter out to a helper function
so that it can be re-used later for CMAKE_XCODE_ATTRIBUTE_*.

Source/cmGlobalXCodeGenerator.h

@@ -153,6 +153,8 @@ private:
   std::string ExtractFlag(const char* flag, std::string& flags);
   std::string ExtractFlagRegex(const char* exp, int matchIndex,
                                std::string& flags);
+  void FilterConfigurationAttribute(std::string const& configName,
+                                    std::string& attribute);
   void SortXCodeObjects();
   // delete all objects in the this->XCodeObjects vector.
   void ClearXCodeObjects();