[Bug libstdc++/86590] Codegen is poor when passing std::string by value with _GLIBCXX_EXTERN_TEMPLATE undefined

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86590

--- Comment #15 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #14)
> That's been asked for repeatedly but will never happen, instead this is
> being added to C++2a:
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0595r1.html
> 
> So we'd do
>  
>       static _GLIBCXX17_CONSTEXPR size_t
>       length(const char_type* __s)
>       {
> #if __cplusplus >= 201703L
>          if (std::is_constant_evaluated())

Hmm, std::is_constant_evaluated would only be declared for C++2a, so we'd need
to use the magic builtin directly here, not the std:: wrapper.