[libc++] Re-enable workaround for pre-ranges CTAD in std::span

See https://reviews.llvm.org/D121626 for details -- this re-enables the
CTAD we removed, since it does break some stuff as well (even though it's
not nearly as bad as the removed constructors fixed by D121626).

(cherry picked from commit 6a7f0551)

Differential Revision: https://reviews.llvm.org/D122201