[dart2wasm] Simplify getter and setter forwarder types

This separates setter, getter, and method forwarder types. Setter
forwarders now only take a receiver and a value arguments. Getter
forwarders take only a receiver argument. Method forwarders are the same
as before: a receiver, lists for type, positional, and named arguments.

This is mainly to allow passing more arguments to the method forwarders
without having to pass dummy values when calling getter and setter
forwarders. For example, the vtable entry to fix #50661.

This also fixes return values of field sets (when there is no setter).

New passing tests:

- language/getter/setter_interceptor_test
- language/if_null/assignment_behavior_runtime_18_test
- language/if_null/assignment_behavior_runtime_20_test
- language/if_null/assignment_behavior_runtime_25_test
- language/optimize/setter_test

Change-Id: I17381fba04f089b974d6c435e4c38dc90b7e5527
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/277986
Reviewed-by: Aske Simon Christensen <askesc@google.com>
Commit-Queue: Ömer Ağacan <omersa@google.com>