[clang] [mlir] [OpenACC][NFCI] Implement 'helpers' for all of the clauses I've used so far (PR #137396)

[Andy Kaylor via cfe-commits]

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

This looks good. I just have a nit repeated many times, and a question about code duplication.

https://github.com/llvm/llvm-project/pull/137396