Typeless expressions in GetEnumeratorInfo

[gafter]

The method ForEachLoopBinder.GetEnumeratorInfo has the following in it:

                    // The null and default literals were caught above, so anything else with a null type is a method group or anonymous function
                    diagnostics.Add(ErrorCode.ERR_AnonMethGrpInForEach, _syntax.Expression.Location, collectionExpr.Display);

But this is just wrong (e.g. (1, null) has no type), and in any case is incredibly fragile to future compiler changes. It should be made robust to new expression kinds that can be null.

[gafter]

Compiling

            foreach (var x in (1, null))
            {
            }

gives

CS0446  Foreach cannot operate on a '(int, <null>)'. Did you intend to invoke the '(int, <null>)'?