Allow DenseSet::iterators to be conveted to and compared with const_iterator

Summary:
This seemed to be an oversight seeing as DenseMap has these conversions.

This patch does the following:
- Adds a default constructor to the iterators.
- Allows DenseSet::ConstIterators to be copy constructed from DenseSet::Iterators
- Allows mutual comparison between Iterators and ConstIterators.

All of these are available in the DenseMap implementation, so the implementation here is trivial.

Reviewers: dblaikie, dberris

Reviewed By: dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28999

llvm-svn: 292879