[SmallSet] Add SmallSetIterator.

This patch adds a simple const_iterator implementation for SmallSet by
delegating to either a SmallVector::const_iterator or
std::set::const_iterator, depending on which storage is used by the
SmallSet.

Reviewers: dblaikie, craig.topper

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D47942

llvm-svn: 334887