Fix a race condition in llvm::sys::path::unique_file: when we end up
failing to create the unique file because the path doesn't exist,
don't fail if someone else manages to create the path before we do.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172032 91177308-0d34-0410-b5e6-96231b3b80d8