[fir] Add fir.convert op conversion from FIR to LLVM IR

Add conversion pattern for the `fir.convert` operation.

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

This patch was previously landed with a truncated version that
was failing the windows buildbot.

Reviewed By: rovka, awarzynski

Differential Revision: https://reviews.llvm.org/D113469



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