<rdar://problem/13551789> Fix yet another race in unique_file.

If the directory that will contain the unique file doesn't exist when
we tried to create the file, but another process creates it before we
get a chance to try creating it, we would bail out rather than try to
create the unique file.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178908 91177308-0d34-0410-b5e6-96231b3b80d8