[flang] Load unlimited polymorphic box the same way as other

Unlimited polymorphic descriptor have a set size and can be loaded the
same way as polymorphic or monomorphic descriptors. The descriptor
code gen as been set in D138587.
Of course, the data hold by those descriptors have an unknown size
at compile time.

Depends on D141383

Reviewed By: jeanPerier, PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D141390