[fir] Add substring to fir.slice operation

This patch adds the substriing information to the fir.slice
operation. This will be used by character operations in later
upstreaming patches.

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

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D112441



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