[flang] Relax the check for polymorphic pointer in storage_size intrinsic

The runtime check was a too strong as the standard says.
> If it is polymorphic it shall not be an undefined pointer.
The check was checking if the pointer was associated.
Remove the check as other compilers do.

Depends on D144643

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D144644