[flang] Add TODO for polymorphic array argument in elemental procedure

The current lowering fails to retrieve the shape of polymorphic array
arguments in elemental procedure reference.
Add a TODO because this is supported in the new HLFIR lowering, and
because the current lowering anyway attempts to re-evaluate the
argument to get its shape, which is not correct if the evaluation
has side effects.

Add a test to ensure this is supported with HLFIR.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D148087