[flang][fir] Add support to mangle/deconstruct namelist group name

Add support to create unique name for namelist group and be able to
deconstruct them.

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

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D110331



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