Add a convenience createUniqueDirectory function.

There are a few valid situation where we care about the structure inside a
directory, but not about the directory itself. A simple example is for unit
testing directory traversal.

PathV1 had a function like this, add one to V2 and port existing users of the
created temp file and delete it hack to using it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185059 91177308-0d34-0410-b5e6-96231b3b80d8