[libc++] Add workaround to avoid breaking users of <span> when <ranges> are disabled

Back in 3a208c68, we implemented the range-based constructor for <span>.
However, in doing so, we removed a previous non-standard constructor that
we provided before shipping <ranges>. Unfortunately, that breaks code that
was relying on a range-based constructor until we ship all of <ranges>.

This patch reintroduces the old non-conforming constructors and tests
that were removed in 3a208c68 and uses them whenever <ranges> is
not provided (e.g. in LLVM 14). This is only a temporary workaround
until we enable <ranges> by default in C++20, which should hopefully
happen by LLVM 15.

The goal is to cherry-pick this workaround back to the LLVM 14 release
branch, since I suspect the constructor removal may otherwise cause
breakage out there, like the breakage I saw internally.

We could have avoided this situation by waiting for C++20 to be finalized
before shipping std::span. For example, we could have guarded it with
something like _LIBCPP_HAS_NO_INCOMPLETE_RANGES to prevent users from
accidentally starting to depend on it before it is stable. We did not
have these mechanisms when std::span was first implemented, though.

Differential Revision: https://reviews.llvm.org/D121626