Test for C++20 p0858 - ConstexprIterator requirements.

[Ville Voutilainen]

On Sat, 1 Jun 2019 at 01:24, Ed Smith-Rowland via libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> Greetings,
>
> Iterators for <array> and <string_view> are usabe in a constexpr context
> since C++2017.
>
> This just adds a compile test to make sure and check a box for C++20
> p0858 - ConstexprIterator requirements.


Those tests don't use the iterators in a constexpr context. To do
that, maybe do those std::copy operations
in a constexpr function and then initialize a constexpr variable with
the result of a call to that function?