How to migrate ggc_alloc_XXX for GCC v8.x (git-20170816)?

[Leslie Zhai]

Hi GCC developers,

GCC v4.6's gengtype will auto-generate Allocators for known structs and 
unions, for example: ggc_alloc_tree2WeakVH for tree2WeakVH 
https://github.com/xiangzhai/dragonegg/blob/master/include/dragonegg/gt-cache-4.6.inc#L24

but gengtype will not auto-generate ggc_alloc_XXX for GCC v6.x or v8.x 
(git-20170816), for example: struct GTY((for_user)) tree2WeakVH 
https://github.com/xiangzhai/dragonegg/blob/master/include/dragonegg/gt-cache-8.0.inc#L1284

As ChangeLog-2014 mentioned:

2014-05-17  Trevor Saunders <tsaunders@mozilla.com>

     ...
     (ggc_alloc): Install the type's destructor as the finalizer if it
     might do something.

Please give me some hint about ggc_alloc migration, thanks a lot!

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