Wrong TupleElementIndex on underlying field for Item1 in long tuple

[jcouv]

Either the TupleElementIndex should be zero in the following test (commented line) or possibly we should not have a TupleElementFieldSymbol wrapping a SubstitutedFieldSymbol (the SubstitutedFieldSymbol should store the extra information, such as element name and location to be a tuple element by itself).

        [Fact]
        public void IndexOfUnderlyingFieldsInTuple9()
        {
            string source = @"
class C
{
    (int, int, int, int, int, int, int, int, int) M() => throw null;
}
";
            var comp = CreateCompilation(source);
            var m = (MethodSymbol)comp.GetMember("C.M");
            var tuple = m.ReturnType;
            Assert.Equal("(System.Int32, System.Int32, System.Int32, System.Int32, System.Int32, System.Int32, System.Int32, System.Int32, System.Int32)",
                tuple.ToTestDisplayString());
            Assert.IsType<ConstructedNamedTypeSymbol>(tuple);

            var item1 = tuple.GetMember<TupleElementFieldSymbol>("Item1");
            Assert.Equal("System.Int32 (System.Int32, System.Int32, System.Int32, System.Int32, System.Int32, System.Int32, System.Int32, System.Int32, System.Int32).Item1",
                item1.ToTestDisplayString());
            Assert.Equal(0, item1.TupleElementIndex);

            var item1Underlying = item1.TupleUnderlyingField;
            Assert.Equal("System.Int32 (System.Int32, System.Int32, System.Int32, System.Int32, System.Int32, System.Int32, System.Int32, System.Int32, System.Int32).Item1",
                item1Underlying.ToTestDisplayString());
            Assert.IsType<SubstitutedFieldSymbol>(item1Underlying);
            Assert.Equal(-1, item1Underlying.TupleElementIndex); // should be zero
...
        }