[flang] Handle parent component of extended derived-type

Parent component refers to the parent derived-type of an extended type.
The parent component is skipped when a specififc component is
referred to. This is fine since all the components in extended type
are available in the type itself. When the parent component is referred,
it need to be taken into account correctly.
This patch fixes the case when the parent component is referred. In a
box, an approriate slice is created or updated to point to the first
component of the parent component. For scalar, a simple conversion to
the parent component type is done.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D134170