[flang] Add TODOs for some FORALL assignments with Pointers and Allocatables

Add TODO for whole array allocatable assignment inside FORALL
Whole allocatable array assignment inside FORALL are otherwise currently
hitting more cryptic asserts.

Add TODO in FORALL assignment when a designator appear with a part ref
that is an allocatable or pointer component (a(i)%pointer%k). The lowering
code does not handle this case well because of the pointer dereference.

Differential Revision: https://reviews.llvm.org/D134440