[Bug libstdc++/65165] missing std::promise::set_value_at_thread_exit

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65165

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It's also documented as missing in previous releases e.g.
https://gcc.gnu.org/onlinedocs/gcc-4.9.2/libstdc++/manual/manual/status.html#status.iso.2011