[CFG] Introduce CFGElementRef, a wrapper that knows it's position in a CFGBlock

Previously, collecting CFGElements in a set was practially impossible, because
both CFGBlock::operator[] and both the iterators returned it by value. One
workaround would be to collect the iterators instead, but they don't really
capture the concept of an element, and elements from different iterator types are incomparable.

This patch introduces CFGElementRef, a wrapper around a (CFGBlock, Index) pair,
and a variety of new iterators and iterator ranges to solve this problem.

I guess you could say that this patch took a couple iterations to get right :^)

Differential Revision: https://reviews.llvm.org/D65196

llvm-svn: 368883