NFC: Change usage of 'DenseSet' to 'DenseSetImpl' in DenseSetImpl::ConstIterator.

Summary:
Change usage of 'DenseSet' to 'DenseSetImpl' in a friend declaration within DenseSetImpl::ConstIterator. 'ConstIterator' was never updated when DenseSet was split into an impl when adding support for DenseSetImpl.

This fixes build errors on MSVC when forward declaring DenseSet as this friend decl does not declare the template arguments as well.

Reviewers: jpienaar

Reviewed By: jpienaar

Subscribers: jpienaar, lebedev.ri, dexonsmith, kristina, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62467

llvm-svn: 361775