[flang] Pointer assignment bounds, section subscript, substring

Resolve bounds in pointer assignment.
Remove TODOs for section-subscript and substring range.
Add tests that verify they are done.

Original-commit: flang-compiler/f18@dc2dd85a9a2b08ca906fd53d11faa2886863d4c4
Reviewed-on: https://github.com/flang-compiler/f18/pull/380