C#: Fix bug in guards logic for foreach loops

[hvitved]

This PR fixes a bug where the guards library incorrectly concluded certain expressions to be non-empty. For example:

void M(string[] args)
{
    foreach (var arg in args)
        ;
    Console.WriteLine(args); // INCORRECT: was considered always empty
}

However, this is still valid:

void M(string[] args)
{
    foreach (var arg in args)
        Console.WriteLine(args); // CORRECT: is always non-empty
}

[hvitved]

CSharp-Differences job: https://jenkins.internal.semmle.com/job/Changes/job/CSharp-Differences/401/. The result changes seem be just "normal wobbliness" and not actually a result of this PR.

[tamasvajk]

LGTM