[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

libcxx/include/module.modulemap.in

@@ -1174,6 +1174,7 @@ module std [system] {
 
     module __mdspan {
       module extents                         { private header "__mdspan/extents.h" }
+      module layout_left                     { private header "__mdspan/layout_left.h" }
       module layout_right                    { private header "__mdspan/layout_right.h" }
       module mdspan_fwd                      { private header "__fwd/mdspan.h" }
     }