Default SetVector to use a DenseSet.

We use to have an odd difference among MapVector and SetVector. The map
used a DenseMop, but the set used a SmallSet, which in turn uses a
std::set.

I have changed SetVector to use a DenseSet. If you were depending on the
old behaviour you can pass an explicit set type or use SmallSetVector.
The common cases for needing to do it are:

* Optimizing for small sets.
* Sets for types not supported by DenseSet.

llvm-svn: 253439