Fix (re-)creation of unittest lit.site.cfg for clang-tools-extra.

This has been hiding really well. Hopefully brings the builders suffering from
outdated lit.site.cfg files back to life.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215575 91177308-0d34-0410-b5e6-96231b3b80d8