This patch introduces a new transformation pass that converts
`acc.serial` constructs into `acc.parallel` constructs with
num_gangs(1), num_workers(1), and vector_length(1).
The transformation is semantically equivalent since an OpenACC serial
region executes sequentially, which is identical to a parallel region
with a single gang, worker, and vector. This unification simplifies
processing of acc regions by enabling code reuse in later compilation
stages.
Co-authored-by: Vijay Kandiah <vkandiah@nvidia.com>
