[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 #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Created attachment 48358
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48358&action=edit
Patch to set __cpp_lib_execution correctly

If/when this bug is fixed we'll want something like this patch so that
__cpp_lib_execution has a different value for C++17 and C++20.