ADT: Add explicit conversions for reverse ilist iterators

Add explicit conversions between forward and reverse ilist iterators.
These follow the conversion conventions of std::reverse_iterator, which
are off-by-one: the newly-constructed "reverse" iterator dereferences to
the previous node of the one sent in.  This has the benefit of
converting reverse ranges in place:
  - If [I, E) is a valid range,
  - then [reverse(E), reverse(I)) gives the same range in reverse order.

ilist_iterator::getReverse() is unchanged: it returns a reverse iterator
to the *same* node.

llvm-svn: 294349