Allow data member splices in designated initializers

[katzdm]

P2996 co-authors are exploring whether splicers of data members should be allowed in designated initializers. One of the examples linked to from P2996R2 has this example, which no implementation currently implements:

constexpr Variant() requires std::is_default_constructible_v<first_type>
    : storage_{ .[: get_nth_nsdm(0) :]{} }
    , index_(0)
{
    std::construct_at(&storage_.[: get_nth_nsdm(0) :]);
}

Note that get_nth_nsdm(0) could (and likely would) be dependent on a template parameter.

[katzdm]

Not planning to adopt this for P2996, and therefore will not implement for the time being.