Fix an inconsistency in treatment of trailing / in path::const_iterator

When using a //net/ path, we were transforming the trailing / into a '.'
when the path was just the root path and we were iterating backwards.
Forwards iteration and other kinds of root path (C:\, /) were already
correct.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202999 91177308-0d34-0410-b5e6-96231b3b80d8