[Bug libstdc++/71960] __glibcxx_assert and Debug Mode checks can't be used in constexpr functions

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71960

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Alternatively, compiler magic which allows the checks to be skipped when used
in a constant expression would allow us to support all valid code, at the
expense of not diagnosing misuses in constant expressions.