[C++] Add a warning for undefined user functions
[C++] Add a warning for undefined user functions

google/kati,danw/kati,danw/kati,shinh/kati,google/kati,danw/kati,shinh/kati,google/kati,google/kati,shinh/kati,shinh/kati,shinh/kati,danw/kati,google/kati,danw/kati,danw/kati,shinh/kati