[libc++][mdspan] Implement default_accessor

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

). default_accessor is the trivial accessor
using plain pointers and reference to element types.

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

Differential Revision: https://reviews.llvm.org/D153935