Fix S2259 FP: Engine does not learn NotNull from LINQ methods

[Tim-Pohlmann]

Most LINQ methods like .Count() do throw when the collection is null. The engine should learn NotNull when a method like this is invoked to avoid FPs (e.g., for S2259).

private void Foo(string[] values)
{
    if (values == null)
    {
        Console.WriteLine("Oops!");
    }
    if (5 < values.Count())         // FN, will throw when values is null; separate issue, see #8908
    {
        _ = values[5].ToString();   // Noncompliant FP, we do not learn NotNull from LINQ methods
    }
}

Related: #8908

[antonioaversa]

Handing over this work to @Tim-Pohlmann, who will bring it to completion.