Resubmit "Fix llvm-lit script generation in libcxx."

After speaking with the libcxx owners, they agreed that this is
a bug in the bot that needs to be fixed by the bot owners, and
the CMake changes are correct.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@313643 91177308-0d34-0410-b5e6-96231b3b80d8