Fix return type for `operator=` if no return type is specified in .clif file.
Fix return type for `operator=` if no return type is specified in .clif file.

Most of the time `operator=` returns a reference to the class.

This is for https://source.corp.google.com/piper///depot/google3/quality/ranklab/future_oss/data_structures/python/ids.clif;l=18;bpv=0;bpt=0.

PiperOrigin-RevId: 436827301

google/clif,google/clif,google/clif