[mlir][openacc] Added custom builder for acc::ParallelOp (#98191)
This change adds a custom builder for `acc::ParallelOp`. This enables
users to only specify the operands they would need for their
`acc::ParallelOp` while building it. They can specify nothing to create
an empty `acc.parallel`, or all of the 11 operands listed
[here](https://mlir.llvm.org/docs/Dialects/OpenACCDialect/#operands-27),
or anywhere in between following the specified order in this custom
builder. Unspecified operands are left empty. Additionally, users can
later set the optional attributes such as `numGangsDeviceType` using the
available attribute setters for `acc::ParallelOp`.