As the name implies (and the spec confirms), this is only ever going to
be an object or "nothing", or "undefined" in the spec. By taking this
literally and updating a check to check for `is_undefined()`, we
introduced a bug - the value was still initialized as an empty value.
Instead, use a pointer to an Object - either we have one, or we don't.
Fixes#8448.
This patch implements the IsSimpleParameterList static semantics for
ordinary function objects.
We now also create an unmapped arguments object for callee contexts
with non-simple parameter lists, instead of only doing it in strict
mode. Covered by test262.
