[Bug libstdc++/71579] type_traits miss checks for type completeness in some traits

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71579

--- Comment #13 from Antony Polukhin <antoshkka at gmail dot com> ---
I meant 
+ noexcept(noexcept(Tp(std::declval<Tp>())))

but now I'm not sure that it would test excactly the same thing.