[Bug libstdc++/94702] std::unsequenced_policy should not be defined for C++17

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94702

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Another bug is that we don't support the __cpp_lib_execution macro, which
should have been updated to indicate this policy is supported.