AddVariableExpressions lacks coverage

[jcouv]

In CSharpProximityExpressionsService.RelevantExpressionsCollector.cs

The code below doesn't seem covered by tests (it used to have an infinite recursion bug).

            private void AddVariableExpressions(
                ExpressionSyntax component,
                IList<string> expressions)
            {
                if (!_includeDeclarations) return;

                switch (component.Kind())
                {
                    case SyntaxKind.TupleExpression:
                        {
                            var t = (TupleExpressionSyntax)component;
                            foreach (ArgumentSyntax a in t.Arguments) AddVariableExpressions(a.Expression, expressions);
                            break;
                        }
                    case SyntaxKind.DeclarationExpression:
                        {
                            var t = (DeclarationExpressionSyntax)component;
                            AddVariableExpressions(t.Designation, expressions);
                            break;
                        }
                }
            }