This PR updates the flang lowering to explicitly implement the OpenACC
rules:
- As per OpenACC 3.3 standard section 2.9.6 independent clause: A loop
construct with no auto or seq clause is treated as if it has the
independent clause when it is an orphaned loop construct or its parent
compute construct is a parallel construct.
- As per OpenACC 3.3 standard section 2.9.7 auto clause: When the parent
compute construct is a kernels construct, a loop construct with no
independent or seq clause is treated as if it has the auto clause.
- Loops in serial regions are `seq` if they have no other parallelism
marking such as gang, worker, vector.
For now the `acc.loop` verifier has not yet been updated to enforce
this.
775ad3e49c