whereis PLUGIN_REGISTER_GGC_CACHES? how to migrate it for GCC v6.x?

[Leslie Zhai]

Hi GCC developers,

There was
PLUGIN_REGISTER_GGC_CACHES

pseudo-events for register_callback in GCC v4.x, but how to migrate it 
for GCC v6.x? there is no  PLUGIN_REGISTER_GGC_CACHES deprecated log in 
ChangeLog-201X nor git log plugin.h... please give me some hint, thanks 
a lot!

--
Regards,
Leslie Zhai - https://reviews.llvm.org/p/xiangzhai/