[PathV2] In llvm::sys::fs::unique_file, make sure it doesn't fall into an infinite loop by constantly trying
to create the parent path.

This can happen if the path is a relative filename and the current directory was removed.
Thanks to Daniel D. for the hint in fixing it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176226 91177308-0d34-0410-b5e6-96231b3b80d8