HPCC-12528 Report error if varstring used in an index

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6705 from ghalliday/issue12531

HPCC-12528 Report error if varstring used in an index

Reviewed-By: Jamie Noss <james.noss@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>