HPCC-11098 Allow COMBINE in a thor child query

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5731 from ghalliday/issue11098

HPCC-11098 Allow COMBINE in a thor child query

Reviewed-By: Jake Smith <jake.smith@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>