[libc++] Add a Lit configuration for running back-deployment tests

This testing configuration links tests against one libc++ shared library,
but runs them against another libc++ shared library. This makes sure that
we can build applications against the libc++ provided in a recent SDK and
back-deploy them to platforms containing older libc++ dylibs.

It also switches the Apple CI script to using that new configuration
instead of the legacy one.

Differential Revision: https://reviews.llvm.org/D119195