Codegen NDV computation.

The HLL update and merge functions are cross-compiled and called in
the codegen'd UpdateSlot() function. (The UdfContext functions are
also cross-compiled so they can be inlined.) This speeds up NDV
calculation 2-3x.

Change-Id: Ia0de5e231e4520097ee1a4df8a3dfda5b1843738
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2732
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins