Implement LWG-4314: Missing move in mdspan layout mapping::operator()

[JMazurkiewicz]

This PR implements changes to [mdspan.layout.left.obs], [mdspan.layout.right.obs], and [mdspan.layout.stride.obs]. Other changes cannot be addressed here, because:

• We don't implement padded layouts,
• We already have call to std::forward in _Mdspan_checked_index_cast:
  

  STL/stl/inc/mdspan

  Lines 1225 to 1237 in da06be8

  	template <class _IndexType, class _OtherIndexType>
  	_NODISCARD constexpr _IndexType _Mdspan_checked_index_cast(_OtherIndexType&& _Idx)
  	noexcept(is_nothrow_constructible_v<_IndexType, _OtherIndexType>) {
  	_STL_INTERNAL_STATIC_ASSERT(is_integral_v<_IndexType> && is_constructible_v<_IndexType, _OtherIndexType>);
  	using _Arg_value_t = remove_cvref_t<_OtherIndexType>;
  	if constexpr (is_integral_v<_Arg_value_t> && !is_same_v<_Arg_value_t, bool>) {
  	_STL_VERIFY(_STD in_range<_IndexType>(_Idx),
  	"Each argument to operator[] must be representable by index_type in order for the pack of arguments to be "
  	"a valid multidimensional index (N4964 [mdspan.mdspan.members]/3).");
  	}
  	return static_cast<_IndexType>(_STD forward<_OtherIndexType>(_Idx));
  	}

• Effect of std::forward in other occurrences of index-cast should not be observable by users.

I think we should close #6209 and mention LWG-4314 in #3807.

[StephanTLavavej]

Thanks, I've updated #3807 as you suggested.