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

CI systems like Green Dragon should use this script so as to make
reproducing errors easy locally.

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@350740 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx