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

[kees at outflux dot net via Gcc-bugs]

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

--- Comment #6 from Kees Cook <kees at outflux dot net> ---
I really want to avoid the changes to sizeof() -- this will confuse a lot of
other things. Sizeof is expected to be a constant expression, for example.

I think the attribute is best since it avoids colliding with anything else.