[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 #28 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Created attachment 44540
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44540&action=edit
Use __builtin_is_constant_evaluated()

I tried to replace the __builtin_constant_p and __constant_string conditions
with Jakub's new __builtin_is_constant_evaluated (see patch) but it made no
difference.