HPCC-9610 Improve implicit projection of child queries

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4670 from ghalliday/issue9610

HPCC-9610 Improve implicit projection of child queries

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>