[Bug libstdc++/78991] std::sort and std::unique can not use std::function with clang++ -std=c++14

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78991

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
How positive you are that this is a libstdc++ bug rather than a clang bug?  It
works correctly with GCC 5.4.0's front-end and GCC 7.0's libstdc++ and
front-end.