Should not warn dereferencing collection after foreach

[cston]

The second foreach in each case should not produce a warning since the collection cannot be null after the first dereference.

[module: System.Runtime.CompilerServices.NonNullTypes]
class Enumerable
{
    public System.Collections.IEnumerator GetEnumerator() => throw null;
}
class Program
{
    static void F1(object[]? x)
    {
        foreach (var i in x) { } // warning: possibly null
        foreach (var i in x) { } // ok
    }
    static void F2(Enumerable? y)
    {
        foreach (var i in y) { } // warning: possibly null
        foreach (var i in y) { } // ok
    }
}

[cston]

See NullableReferenceTypesTests.NotNullAfterDereference_ForEach.

[jcouv]

It looks like I incorrectly adjusted NotNullAfterDereference_ForEach, so it's no longer verifying what we want. I'll rectify the test in my next PR.