[llvm] [mlir][OpenMP] Added `omp.structured_region` operation (PR #68825)

[Kiran Chandramohan via llvm-commits]

https://github.com/kiranchandramohan approved this pull request.

Looks OK. A few comments inline. We will have to add entries in the converstion to llvm pass.

https://github.com/llvm/llvm-project/pull/68825