10.4.4.29 [definite assignment for] ?? expressions

The following test case should “pass” to match compilers:

``` cs
using System;
static class Program
{
    static void Main()
    {
        const string x = "pass";
        string y;
        string z = x ?? y;
        Console.WriteLine(z);
    }
}
```

However, this is not justified by the current language specification.  I recommend we modify the specification to allow this.