HPCC-10544 Add debug code for tracking unique expression instances

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5220 from ghalliday/issue10544

HPCC-10544 Add debug code for tracking unique expression instances

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>