[mlir][openacc] Add acc.serial operation

The acc.serial operation models the OpenACC serial construct.
The serial construct defines a region of a program that is to be
executed sequentially on the current device.
The operation is modelled on the acc.parallel operation and will
receive similar updates when the data operands operations will
be implemented.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D148250