[flang] Lower character related intrinsic

This patch adds lowering for the following character related intrinsics:
- `len`
- `len_trim`
- `lge`, `lgt`, `lle` and `llt`

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

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D121703



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