[flang][hlfir][NFC] catch dynamically optional args with no lowering

[tblah]

Summary

These should be lowered with genOptionalValue as in D154897, but I
haven't found any cases where this code path is actually hit (flang
tests, gfortran test suite), so I don't think it would be testable.
Adding an assertion for if this code path ever becomes live.

[vzakhari]

Thank you, Tom. I will see if it triggers in any test after you merge it.

flang/lib/Lower/ConvertCall.cpp

genIntrinsicRefCore(Fortran::lower::PreparedActualArguments &loweredActuals,
   mlir::Location loc = callContext.loc;
   for (auto arg : llvm::enumerate(loweredActuals)) {
     if (!arg.value()) {
       operands.emplace_back(fir::getAbsentIntrinsicArgument());
       continue;
     }
     if (!argLowering) {
       // No argument lowering instruction, lower by value.
+      assert(!arg.value()->handleDynamicOptional() &&
+             "should use genOptionalValue");
       hlfir::Entity actual = arg.value()->getActual(loc, builder);
       operands.emplace_back(
           Fortran::lower::convertToValue(loc, converter, actual, stmtCtx));
       continue;
     }
     // Helper to get the type of the Fortran expression in case it is a
     // computed value that must be placed in memory (logicals are computed as
     // i1, but must be placed in memory as fir.logical).