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

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.

Differential Revision: https://reviews.llvm.org/D155477

flang/lib/Lower/ConvertCall.cpp

@@ -1345,6 +1345,8 @@ genIntrinsicRefCore(Fortran::lower::PreparedActualArguments &loweredActuals,
     }
     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));