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

[jeanPerier]

Summary

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).

[clementval]

LGTM. Thanks.

flang/lib/Lower/ConvertExpr.cpp

#define GENBIN(GenBinEvOp, GenBinTyCat, GenBinFirOp)                           \
 
   template <typename A>
   CC genSlicePath(const A &x, ComponentPath &components) {
     return genarr(x, components);
   }
 
   CC genarr(const Fortran::evaluate::StaticDataObject::Pointer &,
             ComponentPath &components) {
-    fir::emitFatalError(getLoc(), "substring of static array object");
+    TODO(getLoc(), "substring of static object inside FORALL");
   }
 
   /// Substrings (see 9.4.1)
   CC genarr(const Fortran::evaluate::Substring &x, ComponentPath &components) {
     components.substring = &x;
     return std::visit([&](const auto &v) { return genarr(v, components); },
                       x.parent());
   }