This doesn't seem ideal, perhaps we could just keep the llvm_site_cfg and have
other config (clang and clang-tools-extra) derive their site_cfg from that.
Suggestions/complaints/ideas welcome.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@163171 91177308-0d34-0410-b5e6-96231b3b80d8