unittests: Actually test reverse iterators in Path tests

This re-enables some #if 0'd code (since 2010) in the Path unittests
and makes at least a weak effort at testing sys::path's rbegin/rend.

This change was inspired by some test failures near uses of rbegin and
rend here:

    http://lab.llvm.org:8011/builders/clang-x86_64-linux-vg/builds/3209

The "valgrind was whining" comment looked promising in terms of a
simpler to debug case of the same errors. However, it appears that the
valgrind complaints the comment was referring to are distinct from the
ones in the frontend, since this updated test isn't complaining for me
under valgrind.

In any case, the disabled tests weren't helping anybody.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213125 91177308-0d34-0410-b5e6-96231b3b80d8