Move Thread to shared CoreLib (dotnet/corefx#35462)

Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>

Author: filipnavara

src/System.Private.CoreLib/shared/System/AppDomain.cs

@@ -0,0 +1,13 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace System
+{
+    public sealed class AppDomainSetup
+    {
+        internal AppDomainSetup() { }
+        public string ApplicationBase => AppContext.BaseDirectory;
+        public string TargetFrameworkName => AppContext.TargetFrameworkName;
+    }
+}

src/System.Private.CoreLib/shared/System/AppDomainSetup.cs

@@ -0,0 +1,36 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Runtime.Serialization;
+
+namespace System
+{
+    [Serializable]
+    [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
+    public class CannotUnloadAppDomainException : SystemException
+    {
+        internal const int COR_E_CANNOTUNLOADAPPDOMAIN = unchecked((int)0x80131015); // corresponds to __HResults.COR_E_CANNOTUNLOADAPPDOMAIN in corelib
+        public CannotUnloadAppDomainException()
+            : base(SR.Arg_CannotUnloadAppDomainException)
+        {
+            HResult = COR_E_CANNOTUNLOADAPPDOMAIN;
+        }
+
+        public CannotUnloadAppDomainException(string message)
+            : base(message)
+        {
+            HResult = COR_E_CANNOTUNLOADAPPDOMAIN;
+        }
+
+        public CannotUnloadAppDomainException(string message, Exception innerException)
+            : base(message, innerException)
+        {
+            HResult = COR_E_CANNOTUNLOADAPPDOMAIN;
+        }
+
+        protected CannotUnloadAppDomainException(SerializationInfo info, StreamingContext context) : base(info, context)
+        {
+        }
+    }
+}