C#: Property- and Indexer calls for ref return properties and indexers.

[michaelnebel]

In this PR we improve the call target logic for Properties and Indexers that uses ref types for return. For ref return properties and indexers, it is only possible to declare a getter, which will then also be used in contexts, where a setter would normally be applied (but since the getter returns a reference, it can be used instead). Furthermore, for property and indexers with ref return types, it is only possible declare getters (to avoid ambiguity).

Example.

public ref struct S
{
    private ref int x;

    public S(ref int v1)
    {
        x = ref v1;
    }

    public ref int Prop
    {
        get { return ref x; }
    }
}

public void M()
{
    int a = 0;
    S s = new S(ref a);
    s.Prop = 10; // Uses the getter.
    var y = s.Prop;
}