[fir] Add fircg.ext_embox conversion

Convert a fircg.ext_embox operation to LLVM IR dialect.
A fircg.ext_embox is converted to a sequence of operation that
create, allocate if needed, and populate a descriptor.

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

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D114148



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