Fix test issues from r211623 and remove test-only API

1) missing iterator bits needed by libstdc++4.7
Using find_if was convenient, but since operator++ wasn't a good
interface anyway, I just replaced with a range-based for loop and
removed operator++ from the directory_iterator class.

2) stop relying on order of iterating real files

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211633 91177308-0d34-0410-b5e6-96231b3b80d8