[flang] Fix spurious error on defined assignment in PURE (#139186)
An assignment to a whole polymorphic object in a PURE subprogram that is
implemented by means of a defined assignment procedure shouldn't be
subjected to the same definability checks as it would be for an
intrinsic assignment (which would also require it to be allocatable).
Fixes https://github.com/llvm/llvm-project/issues/139129.