[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).

Differential Revision: https://reviews.llvm.org/D122184