[Bug c++/85523] Add fix-it hint for missing return statement in assignment operators

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85523

--- Comment #4 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
Note to self: this came out of this ML thread:
  https://gcc.gnu.org/ml/gcc/2018-04/msg00168.html