[flang] Lower character related intrinsics

This patch adds lowering for some character related
intrinsics:
- `scan`
- `verify`

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

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D121842



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