[flang][OpenMP][OpenACC] Support stop statement in OpenMP/OpenACC region

[flang][OpenMP][OpenACC] Support stop statement in OpenMP/OpenACC region

This supports lowering of stop statement in OpenMP/OpenACC region.
* OpenMP/OpenACC: Emit `fir.unreachable` only if the block is not
  terminated by any terminator. This avoids knocking off an existing
  OpenMP/OpenACC terminator.
* OpenMP: Emit the OpenMP terminator instead of `fir.unreachable` since
  OpenMP regions can only be terminated by OpenMP terminators. This is
  currently skipped for OpenACC since unstructured code is not yet
  handled specially in OpenACC lowering.

Fixes #60737
Fixes #61877

Co-authored-by: Kiran Chandramohan <kiranchandramohan@gmail.com>
Co-authored-by: Val Donaldson <vdonaldson@nvidia.com>

Reviewed By: vdonaldson, peixin

Differential Revision: https://reviews.llvm.org/D129969