C#: Data flow for positional patterns.

[michaelnebel]

In this PR we add data flow support via positional patterns. That is,

var x = (1, (2, 3), 4);
switch (x)
{
    case (var a1, (var b1, var c1), _):
        Sink(a1);        // has value flow 1
        Sink(c1);        // has value flow 3
        Sink(b1);       // has value flow 2.
        break;
}

Note, that positional patterns are similar to tuple patterns (var (x, y)), but they haven't been handled in the data flow library before now.