[libc++] Fix std::layout_left::stride(r)

[crtrott]

Summary

The function was using the stride calculation for layout_right not layout_left. And the test was also testing for layout_right ...

libcxx/include/__mdspan/layout_left.h

#  endif
   _LIBCPP_HIDE_FROM_ABI constexpr index_type stride(rank_type __r) const noexcept
     requires(extents_type::rank() > 0)
   {
     // While it would be caught by extents itself too, using a too large __r
     // is functionally an out of bounds access on the stored information needed to compute strides
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
         __r < extents_type::rank(), "layout_left::mapping::stride(): invalid rank index");
     index_type __s = 1;
-    for (rank_type __i = extents_type::rank() - 1; __i > __r; __i--)
+    for (rank_type __i = 0; __i < __r; __i++)
       __s *= __extents_.extent(__i);
     return __s;
   }
 
   template <class _OtherExtents>
     requires(_OtherExtents::rank() == extents_type::rank())
   _LIBCPP_HIDE_FROM_ABI friend constexpr bool
   operator==(const mapping& __lhs, const mapping<_OtherExtents>& __rhs) noexcept {

libcxx/test/std/containers/views/mdspan/layout_left/stride.pass.cpp

constexpr void test_stride(std::array<typename E::index_type, E::rank()> strides, Args... args) {
   for (size_t r = 0; r < E::rank(); r++)
     assert(strides[r] == m.stride(r));
 }
 
 constexpr bool test() {
   constexpr size_t D = std::dynamic_extent;
   test_stride<std::extents<unsigned, D>>(std::array<unsigned, 1>{1}, 7);
   test_stride<std::extents<unsigned, 7>>(std::array<unsigned, 1>{1});
-  test_stride<std::extents<unsigned, 7, 8>>(std::array<unsigned, 2>{8, 1});
-  test_stride<std::extents<int64_t, D, 8, D, D>>(std::array<int64_t, 4>{720, 90, 10, 1}, 7, 9, 10);
+  test_stride<std::extents<unsigned, 7, 8>>(std::array<unsigned, 2>{1, 7});
+  test_stride<std::extents<int64_t, D, 8, D, D>>(std::array<int64_t, 4>{1, 7, 56, 504}, 7, 9, 10);
   return true;
 }
 
 int main(int, char**) {
   test();
   static_assert(test());
   return 0;
 }