[flang] Lower command and environment intrinsics

This patch adds lowering for command and environment
related intrinsics:
- `get_command_argument`
- `get_environment_variable`
- `command_argument_count`

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

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D121909



Co-authored-by: Josh Mottley <Josh.Mottley@arm.com>
Co-authored-by: Jean Perier <jperier@nvidia.com>