Fix Windows build warning

lib/Analysis/NaCl/PNaClABIProps.cpp

 //===- PNaClABIProps.cpp - Verify PNaCl ABI Function Rules ----------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // Verify function-level PNaCl ABI properties, at the construct level.
@@ skipping 70 matching lines @@
   case GlobalValue::LinkOnceAnyLinkage:   return "linkonce";
   case GlobalValue::LinkOnceODRLinkage:   return "linkonce_odr";
   case GlobalValue::WeakAnyLinkage:       return "weak";
   case GlobalValue::WeakODRLinkage:       return "weak_odr";
   case GlobalValue::CommonLinkage:        return "common";
   case GlobalValue::AppendingLinkage:     return "appending";
   case GlobalValue::ExternalWeakLinkage:  return "extern_weak";
   case GlobalValue::AvailableExternallyLinkage:
     return "available_externally";
   }
+  llvm_unreachable("unhandled GlobalValue::LinkageTypes");
 }
 
 bool PNaClABIProps::isValidGlobalLinkage(GlobalValue::LinkageTypes Linkage) {
   switch (Linkage) {
     case GlobalValue::ExternalLinkage:
       return true;
     case GlobalValue::InternalLinkage:
       return true;
     default:
       return false;
   }
 }