[fir] !fir.tdesc type conversion

Add !fir.tdesc type conversion.
!fir.tdesc is converted to a llvm.ptr<i8>.

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

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D113769



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