[libc++][mdspan] Implement layout_left

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

). layout_left is a layout mapping policy
whose index mapping corresponds to the memory layout of Fortran arrays.
Thus the left most index has stride-1 access, and the right most index
is associated with the largest stride.

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

Differential Revision: https://reviews.llvm.org/D153783