[Bug c++/125371] Missing fixit hint for forward declaration w/ std::istringstream and missing include

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125371

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
RFC patch: https://gcc.gnu.org/pipermail/gcc-patches/2026-May/717249.html