Use the original arg length of wrapped CP funcs, still call the wrapper

`ComputedPropertyPrototype.set` analyzes the length of CP arguments to decide how the CP function should be called. Since the wrapper (which sets up `_super`) handles arbitrary arguments, the original function (at `wrappedFunction`) should be inspected for `.length`. The wrapper, however, should still be called when performing the set (ensuring that `_super` is present).

Fixes #2890