Lift the NumElements and NumTombstones members into the super class

rather than the base class. Add a pile of boilerplate to indirect around
this.

This is pretty ugly, but it allows the super class to change the
representation of these values, which will be key for doing
a SmallDenseMap.

Suggestions on better method structuring / naming are welcome, but keep
in mind that SmallDenseMap won't have an 'unsigned' member to expose
a reference to... =/

llvm-svn: 158586