[flang] turn fatal error into a TODO for FORALL edge-case

See https://github.com/llvm/llvm-project/issues/59337.
This TODO will be implemented as part of HLFIR work
(see https://github.com/llvm/llvm-project/blob/main/flang/docs/HighLevelFIR.md).

Differential Revision: https://reviews.llvm.org/D139410