[clang] [OpenACC][CIR] Implement basic lowering for combined constructs (PR #139119)

[Razvan Lupusoru via cfe-commits]

razvanlupusoru wrote:

> in that they are the only one where there are two operations for a single construct.

https://mlir.llvm.org/docs/Dialects/OpenACCDialect/#operation-categories
There are other categories where a single construct leads to multiple operations. See description at the link above about the "second group". I should add combined constructs to that list.

https://github.com/llvm/llvm-project/pull/139119