Report warning using foreach with a nullable value

[cston]

A warning should be reported using foreach with a nullable value.

class C
{
    static void F(object[]? args)
    {
        foreach (var o in args) // warn
        {
        }
    }
}

[jcouv]

Also, we need to check the nullability of values returned by yield:

using System.Collections.Generic;
public class C 
{
    public IEnumerable<(string, string)> M() 
    {
       yield return (null, null); // expect warning
    }
}

Example

[cston]

yield is tracked in #23701.