[clang] [mlir][OpenMP] Added omp.region operation (PR #65243)

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shraiysh wrote:

`internal_region` sounds good. I did not want to make it highly specific, because it could be used under other constructs too. Like, `omp.sections` could have an internal region and a barrier in the future. We do not need it for others at the moment, but not making it highly special leaves room for other constructs to use it.

I will create an RFC.

https://github.com/llvm/llvm-project/pull/65243