Add NotNullAttribute and NullableAttribute

Antlr4.Runtime/Antlr4.Runtime.csproj

@@ -119,6 +119,7 @@
     <Compile Include="LexerNoViableAltException.cs" />
     <Compile Include="Misc\Args.cs" />
     <Compile Include="Misc\Array2DHashSet`1.cs" />
+    <Compile Include="Misc\NullableAttribute.cs" />
     <Compile Include="Misc\DoubleKeyMap`3.cs" />
     <Compile Include="Misc\FlexibleHashMap`2.cs" />
     <Compile Include="Misc\IFunc0`1.cs" />
@@ -128,6 +129,7 @@
     <Compile Include="Misc\IntervalSet.cs" />
     <Compile Include="Misc\IPredicate`1.cs" />
     <Compile Include="Misc\MultiMap`2.cs" />
+    <Compile Include="Misc\NotNullAttribute.cs" />
     <Compile Include="Misc\OrderedHashSet`1.cs" />
     <Compile Include="Misc\ParseCanceledException.cs" />
     <Compile Include="Misc\RuleDependencyChecker.cs" />

Antlr4.Runtime/Misc/NotNullAttribute.cs

@@ -0,0 +1,9 @@
+namespace Antlr4.Runtime.Misc
+{
+    using System;
+
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.ReturnValue, Inherited = true, AllowMultiple = false)]
+    public sealed class NotNullAttribute : Attribute
+    {
+    }
+}

Antlr4.Runtime/Misc/NullableAttribute.cs

@@ -0,0 +1,9 @@
+namespace Antlr4.Runtime.Misc
+{
+    using System;
+
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.ReturnValue, Inherited = true, AllowMultiple = false)]
+    public sealed class NullableAttribute : Attribute
+    {
+    }
+}