Support ref field assignment in object initializers

[cston]

ref struct R<T>
{
    public ref T F;
}

int i = 0;
var r = new R<int> { F = ref i };

See LDM notes.

Relates to test plan #59194

[ufcpp]

int x = 1;

var a = new A();
a.X = ref x; // error. (a is not scoped.)

var b = new A { X = ref x }; // no error?

ref struct A { public ref int X; }

[cston]

@ufcpp, the escape scope of the local is set by the initializer unless the local is explicitly declared scoped.

For a, the initializer value new A() can be returned from the current method, so a is unscoped. For b, the initializer cannot escape the current method, so b is implicitly scoped.

The error for a.X = ref x; can be avoided by declaring a as scoped var a = new A();.