[flang][OpenACC] Lower init/shutdown directive

This patch adds lowering for the `!$acc init` and `!$acc shutdown`
from the PFT to OpenACC dialect.

This patch is part of the upstreaming effort from fir-dev branch.

Depends on D122384

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D122387