[mlir][openacc] Add firstprivate representation

Add a representation for firstprivate clause modeled on the
private representation added in D150622.
The firstprivate recipe operation has an additional mandatory
region representing a sequences of operations needed to copy
the initial value to the created private copy.

Depends on D150622

Reviewed By: razvanlupusoru

Differential Revision: https://reviews.llvm.org/D150729