missing operands for "cpp_not_typechecked"

In `cpp_typecheck_compound_type.cpp`, the `op0` of an `exprt` with id `cpp_no_typechecked` is passed to another function, but `cpp_not_typechecked` does not have operands and therefore the call to `front()` returns a `nullptr`.

This is shown for example in `cpp/Decltype2`.