[ADT] Fix zip iterator interface.

This commit provides `zip_{first,shortest}` with the standard member types and
methods expected of iterators (e.g., `difference_type`), in order for zip to be
used with other adaptors, such as `make_filter_range`.

Support for reverse iteration has also been added.

Differential Revision: https://reviews.llvm.org/D30246

llvm-svn: 296036