[fir] Add fir.gentypedesc conversion

Add conversion pattern for the GenTypeDescOp.
Convert to a global constant with an addressof.

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

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D113766



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