[flang][debug] Support ClassType. (#114809)

This PR adds the handling of `ClassType`. It is treated as pointer to
the underlying type. Note that `ClassType` when passed to the function
have double indirection so it is represented as pointer to type
(compared to other types which may have a single indirection).

If `ClassType` wraps a pointer or allocatable then we take care to
generate it as PTR -> type (and not PTR -> PTR -> type).

This is how it looks like in the debugger.

```
subroutine test_proc (this)
    class(test_type), intent (inout) :: this
    allocate (this%b (3, 2))
    call fill_array_2d (this%b)
    print *, this%a
end
```

```
(gdb) p this
$6 = (PTR TO -> ( Type test_type )) 0x2052a0
(gdb) p this%a
$7 = 0
(gdb) p this%b
$8 = ((1, 2, 3) (4, 5, 6))

```