Use llvm_unreachable to quiet a VC++ compiler warning.

lldb/trunk/source/Expression/DiagnosticManager.cpp

 //===-- DiagnosticManager.cpp -----------------------------------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #include "lldb/Expression/DiagnosticManager.h"
+
+#include "llvm/Support/ErrorHandling.h"
+
 #include "lldb/Core/Log.h"
 #include "lldb/Core/StreamString.h"
 
 using namespace lldb_private;
 
 void
 DiagnosticManager::Dump(Log *log)
 {
switch (severity)
         // this should be exhaustive
         case lldb_private::eDiagnosticSeverityError:
             return "error: ";
         case lldb_private::eDiagnosticSeverityWarning:
             return "warning: ";
         case lldb_private::eDiagnosticSeverityRemark:
             return "";
     }
+    llvm_unreachable("switch needs another case for DiagnosticSeverity enum");
 }
 
 std::string
 DiagnosticManager::GetString(char separator)
 {
     std::string ret;
 
     for (const Diagnostic *diagnostic : Diagnostics())