Add GetTypeDesc() method to FunctionContext.

This is currently only implemented for NativeUdfExpr.

Change-Id: I81b442c5668dff43d0486d1cfc445bca2af66606
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1664
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins