[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 #29 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The seemingly-redundant 'else' keywords in the patch are needed because of PR
86678.