Fix missing constexpr and device annotation for mapping ctors

[tpadioleau]

Add missing constexpr and device annotation. Quoting p2642

template<class LayoutLeftPaddedMapping>
constexpr explicit(! is_convertible_v<typename LayoutLeftPaddedMapping::extents_type, extents_type>)
mapping(const LayoutLeftPaddedMapping&) noexcept;

And

template<class LayoutLeftPaddedMapping>
constexpr explicit(! is_convertible_v<typename LayoutLeftPaddedMapping::extents_type, extents_type>)
mapping(const LayoutLeftPaddedMapping& other) noexcept;