Prepare for an Error Prone flag handling change
Prepare for an Error Prone flag handling change

Previously -Xep:CheckReturnValue:OFF disabled CheckReturnValue
and the separate ReturnValueIgnored check. In the future checks
will only be suppressible using their 'canonical' name.

PiperOrigin-RevId: 150023461

google/j2cl,google/j2cl,google/j2cl,google/j2cl,google/j2cl