[libc++][mdspan] Implement layout_right

This commit implements layout_right in support of C++23 mdspan
(https://wg21.link/p0009

). layout_right is a layout mapping policy
whose index mapping corresponds to the memory layout of multidimensional
C-arrays, and is thus also referred to as the C-layout.

Co-authored-by: Damien L-G <dalg24@gmail.com>

Differential Revision: https://reviews.llvm.org/D151267