HPCC-9003 Check inside expression attributes for evaluation in thor.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4147 from ghalliday/issue9003

HPCC-9003 Check inside expression attributes for evaluation in thor.

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>