[flang] Use derivedType from toAddedum to get updated components

When the rhs is polymorphic and allocated during assignment, the
derivedType might have change from the one set in `toDerived`.
Use the one set in the addendum so it is always up to date.

This can happen in cases like the one shown below:

```
type :: t1
end type t1

type, extends(t1) :: t2
  integer, allocatable :: i(:)
end type

subroutine assign(t)
  class(t2), intent(in) :: t
  class(t1), allocatable :: cp

  cp = t
end subroutine
```

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D144171