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

[kees at outflux dot net]

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

--- Comment #1 from Kees Cook <kees at outflux dot net> ---
Matching Clang feature request:
https://github.com/llvm/llvm-project/issues/99774