[libcxx] Add a script to run CI on older MacOS versions

This script can be used by CI systems to test things like availability
markup and binary compatibility on older MacOS versions. This is still
a bit rough on the edges, for example we don't test libc++abi yet.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@350752 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed