[libc++] Add CI without experimental features and don't exclude span from the tests

There is no reason for the parts of std::span that don't depend on ranges
to be disabled when ranges aren't provided. Also, to make sure the
"no-experimental-stuff" configuration is tested, add a CI job for it.

Differential Revision: https://reviews.llvm.org/D118740

(cherry picked from commit 99ae4582)