[Bug c/83407] Warn about unsigned size_t underflow

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83407

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
overflow/underflow for unsigned integer type is well-defined in C so I'm not
sure how we could warn without warning about legit code.