Towards #1485.

On self-assignment, including self-move-assignment, any resets to empty, then calls _Move_from on itself, which treats empty any as _Any_representation::_Trivial and does self-memcpy.

Self-memcpy does seem unacceptably squirrely.

There are number of ways to fix this issue. We can do this check directly in operator=s, to avoid any other potential impact. Or we can do the check for empty any directly in _Move_from, to optimize other empty any assignment in addition.

I've arbitrarily chosen to do this in _Assign, to keep ruining the any that is being self-assigned, do it in just one place, and not do any changes beyond the needed.