[specialized.algorithms.general] Make deref-move constexpr

[cpplearner]

std::uninitialized_move and std::uninitialized_move_n are constexpr and invoke deref-move, but deref-move is not constexpr. This looks like an obvious mistake.

[tkoeppe]

This does not look editorial to me, even if it's obvious. @jwakely: what do you think?

[frederick-vs-ja]

Background: P3508R0 and LWG3918, both touching std::uninitialized_move(_n), were adopted at the same meeting, and unfortunately none of them was aware of the other.

[tkoeppe]

Background: P3508R0 and LWG3918, both touching std::uninitialized_move(_n), were adopted at the same meeting, and unfortunately none of them was aware of the other.

That sounds like a textbook example of a case for an LWG issue!

But let's see if @jwakely considers this obvious enough to be editorial -- it might be, it's just not a call I want to make on my own.

[jwakely]

I've created LWG4452