[Bug c/68120] can't easily deal with integer overflow at compile time

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68120

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
I raised c++/70507 to make a record of another problem caused by the built-ins
not being treated as constant expressions.