Fix #4016: Don't consume nullable flags for anonymous type arguments

Roslyn writes [Nullable] for an anonymous type as if the type was not
generic, so its type arguments have no flags. Taking one per argument
shifted every annotation after the anonymous type, which showed on the
cached lambda fields in <>c, and tripped the dummy-flag assert when the
shifted stream landed on a generic struct. [Dynamic] and
[TupleElementNames] do have entries for those arguments (checked against
what Roslyn emits), so only the nullable index skips them.

ICSharpCode.Decompiler.Tests/TestCases/IsolatedDecompilation/AnonymousTypeNullability.cs

@@ -0,0 +1,33 @@
+#nullable enable
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace ICSharpCode.Decompiler.Tests.TestCases.IsolatedDecompilation
+{
+	public class AnonymousTypeNullability
+	{
+		public void ReferenceTypeAfterAnonymousType(IEnumerable<string> xs)
+		{
+			xs.Select(x => new { A = x, B = x }).Pick(a => new List<string?>());
+		}
+
+		public void ValueTypeAfterAnonymousType(IEnumerable<string> xs)
+		{
+			xs.Select(x => new { A = x }).PickPair(a => new KeyValuePair<string, string?>(a.A, null));
+		}
+	}
+
+	public static class AnonymousTypeNullabilityExtensions
+	{
+		public static IEnumerable<T> Pick<TSource, T>(this IEnumerable<TSource> source, Func<TSource, List<T?>> selector) where T : class
+		{
+			throw new NotSupportedException();
+		}
+
+		public static IEnumerable<T> PickPair<TSource, T>(this IEnumerable<TSource> source, Func<TSource, KeyValuePair<string, T?>> selector) where T : class
+		{
+			throw new NotSupportedException();
+		}
+	}
+}