[mlir][openacc] Add separate acc data operations for OpenACC data clauses

As outlined in [1], data clauses are now implemented as separate operations
from the constructs that they belong to. Some of the highlighted benefits:
- Correctly represent dataflow of data operations
- Easier to track debugging information
- Friendlier to add attributes and to optimize operations

For now, all of the other operand lists are being kept until all references
to them in LLVM can be removed (such as those in flang lowering)

[1] https://discourse.llvm.org/t/rfc-openacc-dialect-data-operation-improvements/69825

Reviewed By: clementval, vzakhari

Differential Revision: https://reviews.llvm.org/D148389