[fir] Add substr information to fircg.ext_embox and fircg.ext_rebox operations

This patch adds the substring information to the fircg.ext_embox and
fircg.ext_rebox operations.

Substring is used for CHARACTER types.

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

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D112807



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