HPCC-11441 Fold IF() at parse time to avoid binding issues later

* Note this also speeds parsing up since the expression trees are smaller

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5908 from ghalliday/issue11441

HPCC-11441 Fold IF() at parse time to avoid binding issues later

Reviewed-By: Jamie Noss <james.noss@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>