[clang] [OpenACC][CIR] Start work to lower 'loop' (PR #137972)

[Bruno Cardoso Lopes via cfe-commits]

bcardosolopes wrote:

> In the future, we'll have to teach the OpenACC dialect how to derive the upperbound, lowerbound, and step from the cir.for loop

Yea, even better if the `cir.for` representation can itself be improved to expose that information (either by transforming it into a more refined loop op or by redesigning the existing one).

https://github.com/llvm/llvm-project/pull/137972