[Flang] Add type conversion for FIR heap type

Convert fir.heap type to its llvm equivalent type (llvm.ptr)

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

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D113670



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