chore: import upstream snapshot with attribution

dotnet/src/LegacySupport/DiagnosticClasses/README.md

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+# DiagnosticClasses
+
+To use this source in your project, add the following to your `.csproj` file:
+
+```xml
+<PropertyGroup>
+  <InjectDiagnosticClassesOnLegacy>true</InjectDiagnosticClassesOnLegacy>
+</PropertyGroup>
+```

dotnet/src/LegacySupport/DiagnosticClasses/UnreachableException.cs

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+// Copyright (c) Microsoft. All rights reserved.
+
+// Polyfill for using UnreachableException with .NET Standard 2.0
+
+namespace System.Diagnostics;
+
+#pragma warning disable CA1064 // Exceptions should be public
+#pragma warning disable CA1812 // Internal class that is (sometimes) never instantiated.
+
+/// <summary>
+/// Exception thrown when the program executes an instruction that was thought to be unreachable.
+/// </summary>
+internal sealed class UnreachableException : Exception
+{
+    private const string MessageText = "The program executed an instruction that was thought to be unreachable.";
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="UnreachableException"/> class with the default error message.
+    /// </summary>
+    public UnreachableException()
+        : base(MessageText)
+    {
+    }
+
+    /// <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 ?? MessageText)
+    {
+    }
+
+    /// <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 ?? MessageText, innerException)
+    {
+    }
+}