You're not supposed to literally type "foo"; it's a placeholder word, to be replaced with whatever port you're having trouble compiling with clang—in this case "llvm-3.1". However, in this case, you don't need to pursue this further, since we already know that llvm-3.1 should build just fine with clang; the problem is something else.
Your log contains the words "Skipping completed", which mean this was not a clean attempt. Please run "sudo port clean llvm-3.1" and then try again, and attach the new main.log if it fails again.
