PathV2: Fix a possible infinite loop.

PathV2: Fix a possible infinite loop.

 - The code could infinite loop trying to create unique files, if the directory
   containing the unique file exists, but open() calls on non-existent files in
   the path return ENOENT. This is true on the /dev/fd filesystem, for example.

 - Will add a clang side test case for this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@168081 91177308-0d34-0410-b5e6-96231b3b80d8