[Bug c++/87409] Implement -Wunused-private-field

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87409

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note removal of private fields can change the ABI of a structure so this
warning should never be turned on by -Wextra or -Wall.