[fir] Add fircg.ext_array_coor conversion

This patch adds the conversion pattern for the fircg.ext_array_coor
operation. It applies the address arithmetic on a dynamically shaped, shifted
and/or sliced array.

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

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D113968



Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>
Co-authored-by: Jean Perier <jperier@nvidia.com>