[clang2ir] do not warn about unknown option: assume clang will handle it

FramaCIRGen.cpp

@@ -203,7 +203,7 @@ ProcessArguments::process(char** argument, int& currentArgument) {
           _doesGenerateImplicitMethods=true;
           --currentArgument;
         } else {
-          std::cout << "Ignore unknown argument: " << argument[0] << std::endl;
+          // Let clang handle it directly
           --currentArgument;
         }
         return true;