[flang][openacc] Lower parallel loop combined construct

[clementval]

Summary

This patch lower the parallel loop combined construct to MLIR ops.

This code was part of the PR #558 on fir-dev
https://github.com/flang-compiler/f18-llvm-project/pull/558

[schweitz]

LGTM.

flang/lib/Lower/OpenACC.cpp

615

When we get the green-light to upstream, you may want to consider using fir::emitFatalError() here, as it will take mlir::Location information and produce a bit nicer output.

(On the other hand, the error seems like a general "this can never happen", but this was a chance to add a gratuitous plug.)

[mehdi_amini]

We're waiting on the ability to have tests for these I believe.

[clementval]

In D92065#2478345, @mehdi_amini wrote:

We're waiting on the ability to have tests for these I believe.

Yeah I don't mind waiting for the fir-dev RFC to go through and adding test for that.