swift234

[arc] Move some code that is STL like from ImmutablePointerSet.h -> STLExtras.h

Specifically:

1. set_union_for_each. This is like std::for_each, but it visits its inputs
(which are assumed to be sorted/uniqued sets) in set_union order.
2. is_uniqued_and_sorted.

I also fixed several nits from Jordan.