[flang] Allow pointer association between derived-type pointer and unlimited polymorphic target

Pointer association to unlimited polymorphic target is allowed for
unlimited polymorphic pointer and non-extensible derived-type.
This is checked by the semantic and this patch allows it in the
fir.rebox operation.

Reviewed By: jeanPerier, PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D142104