[Bug c/116016] enhancement: add __builtin_set_counted_by(P->FAM, COUNT) or equivalent

[qinzhao at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116016

--- Comment #17 from qinzhao at gcc dot gnu.org ---
(continue with the comment #16):
the compiler's responsibility is:
A. check whether p->fam has counted-by attribute or not;
B. get the corresponding counted-by field for p->fam and set it the value;

this is mainly to resolve the source code problem as Kees explained in the
description part. 
no need to make it more complicate than necessary.