Fix bugs in recursive_directory_iterator::increment(ec) implementation and tests.

r273060 didn't completely fix the issues in recursive_directory_iterator and
the tests. This patch follows up with more fixes

* Fix bug where recursive_directory_iterator::increment(ec) did not reset
  the error code if no failure occurred.

* Fix bad assertion in the recursive_directory_iterator::increment(ec) test
  that would only fire for certain iteration orders.

llvm-svn: 273070