[fir] Add fir.embox conversion

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

Current limitiation: alignment is set by default but should be retrieved in the specific target.

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

Reviewed By: kiranchandramohan, awarzynski

Differential Revision: https://reviews.llvm.org/D113756



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