Rewrite this class, making the following improvements:

  1. It now actually uses tarjan's algorithm, so it is a efficient inverse
     ackerman's function for union operations, not linear time.
  2. It now stores one copy of the data in the set instead of two.
  3. It now works for elements other than pointers.
  4. It now has a more STL-like interface that exposes iterators instead
     of internal implementation details.

llvm-svn: 20677