Nullability rewriter is O(2^n) when visiting converted tuple literals

[gafter]

The generated Nullability rewriter contains a method that begins

        public override BoundNode VisitConvertedTupleLiteral(BoundConvertedTupleLiteral node)
        {
            BoundTupleLiteral sourceTuple = (BoundTupleLiteral)this.Visit(node.SourceTuple);
            ImmutableArray<BoundExpression> arguments = this.VisitList(node.Arguments);

The bound trees in node.Arguments share part of the expression tree with node.SourceTuple. Consequently, they will be visited twice. When tuples are nested n deep, nodes at the leaves will be visited O(2ⁿ) times.

We want to avoid repeated visits of a given node, so this needs to be revised.

/cc @chsienki

[gafter]

I believe the "original tuple" is not needed. However, the "natural (unconverted) type" of the tuple is needed.