Factor DenseMap into a base class that implements the hashtable logic,

and a derived class that provides the allocation and growth strategy.

This is the first (and biggest) step toward building a SmallDenseMap
that actually behaves exactly the same as DenseMap, and supports all the
same types and interface points with the same semantics.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158585 91177308-0d34-0410-b5e6-96231b3b80d8