Use a vector of unique_ptrs to fix a memory leak introduced in r208179.

Also removed an inaccurate comment that stated that a DenseMap was used as
storage for the ListInit*'s. It's currently using a FoldingSet.

I expect there's a better way to fix this but I haven't found it yet. FoldingSet
is incompatible with the Pool template and I'm not sure if FoldingSet can be
safely replaced with a DenseMap of computed FoldingSetID's to ListInit*'s.