[mlir][openacc] Add gang dim operand to acc.loop operation

OpenACC 3.3 introduces a dim argument on the gang clause. This patch
adds a new operand for it on the acc.loop and update the custom
gang clause parser/printer for it.

Depends on D151970

Reviewed By: razvanlupusoru, jeanPerier

Differential Revision: https://reviews.llvm.org/D151971