Add new System.Diagnostics.UnreachableException polyfill

The System.Diagnostics.UnreachableException.cs file was not actually auto-generated (see Sergio0694#88).

Fixes Sergio0694#60

src/PolySharp.SourceGenerators/EmbeddedResources/System.Diagnostics.UnreachableException.cs

@@ -0,0 +1,42 @@
+// <auto-generated/>
+#pragma warning disable
+#nullable enable annotations
+
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Diagnostics
+{
+    /// <summary>
+    /// Exception thrown when the program executes an instruction that was thought to be unreachable.
+    /// </summary>
+    public sealed class UnreachableException : Exception
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="UnreachableException"/> class with the default error message.
+        /// </summary>
+        public UnreachableException()
+            : base("The program executed an instruction that was thought to be unreachable.")
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="UnreachableException"/> class with a specified error message.
+        /// </summary>
+        /// <param name="message">The error message that explains the reason for the exception.</param>
+        public UnreachableException(string? message)
+            : base(message)
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="UnreachableException"/> class with a specified error message and a reference to the inner exception that is the cause of this exception.
+        /// </summary>
+        /// <param name="message">The error message that explains the reason for the exception.</param>
+        /// <param name="innerException">The exception that is the cause of the current exception.</param>
+        public UnreachableException(string? message, Exception? innerException)
+            : base(message, innerException)
+        {
+        }
+    }
+}

tests/PolySharp.Tests/LanguageFeatures.cs

@@ -1,6 +1,8 @@
 using System;
+using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
+using System.Runtime.ExceptionServices;
 using System.Runtime.Versioning;
 #if !NETSTANDARD2_1
 using System.Threading.Tasks;
@@ -36,6 +38,13 @@ public void Throws([DoesNotReturnIf(true)] bool value)
     {
     }
 
+    // UnreachableException
+    public object Unreachable()
+    {
+        ExceptionDispatchInfo.Capture(new Exception()).Throw();
+        throw new UnreachableException();
+    }
+
     // MaybeNullAttribute
     [return: MaybeNull]
     public string ModifyValue()