HPCC-11464 Default string stored definitions to variable length

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
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 #6042 from ghalliday/issue11605

HPCC-11605 Cherry pick fixes for IF() folding issues

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>