[flang][openacc] Add parser support for the force modifier in the collapse clause

This patch adds parser support for the force modifier on the collapse clause
introduced in OpenACC 3.3.
Lowering will currently hit a TODO as the MLIR representation of the acc.loop
might need some update.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D151974