[Bug sanitizer/63806] #UBSAN ignores signed char possible overflow

[y.gribov at samsung dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63806

--- Comment #4 from Yury Gribov <y.gribov at samsung dot com> ---
Looks like some compilers check integer demotions (e.g. MS checks with their
/RTCc flag).  I wonder if it makes sense to add an optional flag for this
(obviously not enabled under normal -fsanitize=undefined).