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

[Erich Keane via cfe-commits]

erichkeane wrote:

This is my first run at this, so @clementval and @razvanlupusoru : feel free to rename/reword/suggest improvements.  But this makes the lowering implementation of most clauses that I've done so far fairly trivial.


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