nobody includes llvm/ADT/iterator.h any more,

nobody includes llvm/ADT/iterator.h any more,
so get rid of this monstrosity. iterator.h.in is scheduled for deletion in my working copy,
but I wait till I see that configure gets regenerated, as it depends on it. I'll commit
then.
There are still some AC_* tests in the configure.ac dealing with iterators, those can
be zapped probably too.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@80147 91177308-0d34-0410-b5e6-96231b3b80d8