[clang] [mlir] [CIR][OpenACC] Implement 'routine' lowering + seq clause (PR #170207)

[Erich Keane via cfe-commits]

erichkeane wrote:

> Looks great to me!
> 
> Regarding the testing, one aspect I noticed, is that all of the tests exercise with "seq" only. I wanted to point out that omission of "seq" when the user does not specify it is important (and your current implementation seems correct since it only emits this in explicit cases). Lack of seq (or any other explicit parallelism marking) is useful to ensure that compiler can automatically determine parallelism.

Yes, I've only emitted 'seq' so far.  The standard requires exactly 1 of `gang`, `worker`, `vector` or `seq`, which the CFE requires (which is why I required 1 here), so we'll set ONE of those.  

As far as `automatically determine parallelism`, we can relax the Sema check in the future, at which point this would set nothing.

https://github.com/llvm/llvm-project/pull/170207