[flang][openacc] Skeleton for OpenACC construct lowering

Summary:
This patch introduce the basic infrastructure to be able to lower
OpenACC constructs to the future OpenACC dialect.

Reviewers: schweitz, kiranchandramohan, DavidTruby, sscalpone, jdoerfert, ichoyjx

Reviewed By: ichoyjx

Subscribers: ichoyjx, SouraVX, mgorny, jfb, sstefan1, llvm-commits

Tags: #llvm, #flang

Differential Revision: https://reviews.llvm.org/D84195