[libc++] Add a CI job to backdeploy to macOS 10.14

It adds coverage for back-deploying to a system that contains the
filesystem library, which 10.9 (currently our only back-deployment
target in the CI) does not have.

Differential Revision: https://reviews.llvm.org/D92794

GitOrigin-RevId: 8726f94cc74ff1e91a24cb7259e209995283ae0f
1 file changed