Nested tuple literal should have a type and converted type

[jcouv]

In the following test, the tuple literal (1, 2) should have a type and a converted type

        [Fact]
        public void TypeMergingSuccess3()
        {
            string source = @"
class C
{
    static void Main()
    {
        (string x1, var x2) = (null, (1, 2));
        System.Console.WriteLine(x1 + "" "" + x2);
    }
}
";

#12526 (comment)

[jcouv]

Turns out this was a bug in the test code (looking at the argument syntax node, instead of the tuple literal syntax node. Updated TypeMergingSuccess3.