Adding const overloads of operator* and operator-> for DenseSet iterators

This fixes some problems when building ClangDiagnostics.cpp on Visual Studio 2017 RC. As far as I understand, there was a change in the implementation of the constructor for std::vector with two iterator parameters, which in our case causes an attempt to dereference const Iterator objects. Since there was no overload for a const Iterator, the compile would fail.

Patch by Hugo Puhlmann!

Differential Revision: https://reviews.llvm.org/D28726

llvm-svn: 292034