C#: Add missing CFG edges for nested finally blocks

[hvitved]

In nested finally blocks such as

void M12(bool b1, bool b2)
{
    try
    {
        try
        {
            if (b1)
                throw new ExceptionA();
        }
        finally
        {
            try
            {
                if (b2)
                    throw new ExceptionA();
            }
            finally
            {
                Console.WriteLine("Inner finally");
            }
        }
        Console.WriteLine("Mid finally");
    }
    finally
    {
        Console.WriteLine("Outer finally");
    }
    Console.WriteLine("Done");
}

we were unable to distinguish whether the inner finally block inherited an ExceptionA completion from the outer throw or the inner throw. The (somewhat surprising) effect was that we were missing a CFG edge (highlighted):

The solution is to record information about the nesting level of inherited completions, which enables us to distinguish the two cases. Commit-by-commit review is encouraged.

https://jenkins.internal.semmle.com/job/Changes/job/CSharp-Differences/670/

[tamasvajk]

LGTM