Check that the directory does not exist.
Otherwise, it could allows local users to obtain sensitive information or
overwrite arbitrary files via a symlink attack on temporary directories with
predictable names. 

Reported as CVE-2014-2893 ( https://security-tracker.debian.org/tracker/CVE-2014-2893 )
Found by Jakub Wilk



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211051 91177308-0d34-0410-b5e6-96231b3b80d8