[Bug c/108896] provide "element_count" attribute to give more context to __builtin_dynamic_object_size() and -fsanitize=bounds

[muecker at gwdg dot de via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896

--- Comment #10 from Martin Uecker <muecker at gwdg dot de> ---

And to get bounds checking for the flexible array member today,
one could use a macro to attach the bound back to the array
on member access.

https://godbolt.org/z/GbaoYrhav

But the bound from the type is not fed back  into
 __builtin_dynamic_object_size(), so this currently
only helps for direct array refs. But this is something we
may be able to improve.