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

That has been fixed in g:aac39307e8f589dba231083a3e9aa3d0ff302837 so that
__cpp_lib_execution is always defined to 201902L even in C++17, to reflect that
this policy is always defined. That way the feature test macro accurately
describes our implementation.