HPCC-10799 FIX potential invalid C++ for EXISTS(a+b)

Prevent a translated expression from being re-translated

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5358 from ghalliday/issue10799

HPCC-10799 FIX potential invalid C++ for EXISTS(a+b)

Reviewed-By: Jamie Noss <james.noss@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>