Implement nullability analysis in LINQ queries

[jcouv]

One scenario in particular should work: from item in list where item is not null should result in a collection of not-null items. Reported a few times (including here).

Look for references to this issue in the code, such as NullableWalker.VisitQueryClause

Also replaced some PROTOTYPE markers with a reference to this issue:

        // https://github.com/dotnet/roslyn/issues/29863 Record in the node whether type
        // arguments were implicit, to allow for cases where the syntax is not an
        // invocation (such as a synthesized call from a query interpretation).
        private static bool HasImplicitTypeArguments(BoundExpression node)
        {
            var syntax = node.Syntax;
            if (syntax.Kind() != SyntaxKind.InvocationExpression)
            {
                // Unexpected syntax kind.
                return false;
            }
            var nameSyntax = Binder.GetNameSyntax(((InvocationExpressionSyntax)syntax).Expression, out var _);
            if (nameSyntax == null)
            {
                // Unexpected syntax kind.
                return false;
            }
            nameSyntax = nameSyntax.GetUnqualifiedName();
            return nameSyntax.Kind() != SyntaxKind.GenericName;
        }


        public override BoundNode VisitRangeVariable(BoundRangeVariable node)
        {
            var result = base.VisitRangeVariable(node);
            SetResult(node); // PROTOTYPE(NullableReferenceTypes)
            return result;
        }

This was also reported here.