[Bug middle-end/116989] __builtin_clz{,g}/__builtin_ctz{,g} documentation for 0 should be clearer

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116989

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note UBSan already treats it as undefined behavior rather than the result being
undefined.