llvm dev - Nov 2015 - SetVector now defaults to DenseSet

We used 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.

Cheers,
Rafael