[Bug c/66963] __builtin_constant_p and __builtin_choose_expr do not agree on what is a constexpr with -O2

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66963

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I thought this is documented somewhere but __builtin_choose_expr only really
accept constant literals and not constexprs.