[Bug libstdc++/86008] std::quoted(std::basic_string_view) is missing

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86008

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Thanks for checking! For now I'll just keep the same _Quoted_string definition
so it stores a reference to a basic_string_view. We can reconsider before the
GCC 9 release if needed.