[fir] Add !fir.len type conversion

This patch adds the !fir.len type conversion. The type is converted
to the a 32 bits integer.

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

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

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D113658