`<ranges>`: `lazy_split_view` should use `_Non_propagating_cache` instead of `_Defaultabox`

Revealed by libc++ tests std/ranges/range.adaptors/range.lazy.split/ctor.copy_move.pass.cpp and std/ranges/range.adaptors/range.lazy.split/range.lazy.split.inner/iter_swap.pass.cpp.

[range.lazy.split.view] says lazy_split_view<V, Pattern> stores a non-propagating-cache<<iterator_t<V>> if forward_range<V> is false, but MSVC STL's lazy_split_view stores a _Defaultabox:

https://github.com/microsoft/STL/blob/a8888806c6960f1687590ffd4244794c753aa819/stl/inc/ranges#L4612-L4616

This makes lazy_split_view non-copyable when iterator_t<V> is non-copyable.

#include <ranges>
#include <cassert>
using namespace std;

struct MoveOnlyIter {
    using difference_type = int;
    using value_type = int;

    MoveOnlyIter(MoveOnlyIter&&);
    MoveOnlyIter& operator=(MoveOnlyIter&&);

    int operator*() const;
    MoveOnlyIter& operator++();
    void operator++(int);

    bool operator==(int) const;
};

struct TestRange {
    MoveOnlyIter begin();
    int end();
};

void test() {
    TestRange r;
    auto lsv = r | std::views::lazy_split(0);
    auto lsv2 = lsv;  // error on MSVC, should be OK
}

This seems easy to fix, but might require an ABI-breaking change: both _Defaultabox and _Non_propagating_cache have space-efficient specializations, but with different constraints, which means they may have different layout in some cases.

Start in stl/inc/ranges.html at the lazy_split_view storage referenced by the issue, then compare _Defaultabox with _Non_propagating_cache and their space-efficient specializations. Run std/ranges/range.adaptors/range.lazy.split/ctor.copy_move.pass.cpp and range.lazy.split.inner/iter_swap.pass.cpp; done means the move-only iterator case is copyable and the ABI/layout implications are understood.