Fix Windows build warning

lib/Bitcode/NaCl/Reader/NaClBitstreamReader.cpp

 //===- NaClBitstreamReader.cpp --------------------------------------------===//
 //     NaClBitstreamReader implementation
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
@@ skipping 139 matching lines @@
     report_fatal_error("Not to be used with literals!");
   case NaClBitCodeAbbrevOp::Fixed:
     return Read((unsigned)Op.getValue());
   case NaClBitCodeAbbrevOp::VBR:
     return ReadVBR64((unsigned)Op.getValue());
   case NaClBitCodeAbbrevOp::Array:
     report_fatal_error("Bad array abbreviation encoding!");
   case NaClBitCodeAbbrevOp::Char6:
     return NaClBitCodeAbbrevOp::DecodeChar6(Read(6));
   }
+  llvm_unreachable("unhandled NaClBitCodeAbbrevOp encoding");
 }
 
 void NaClBitstreamCursor::readArrayAbbrev(
     const NaClBitCodeAbbrevOp &Op, unsigned NumArrayElements,
     SmallVectorImpl<uint64_t> &Vals) {
   for (; NumArrayElements; --NumArrayElements) {
     Vals.push_back(readArrayAbbreviatedField(Op));
   }
 }
 
@@ skipping 181 matching lines @@
         if (Record.size() < 1) return true;
         CurBlockInfo = &BitStream->getOrCreateBlockInfo((unsigned)Record[0]);
         if (Listener) {
           Listener->Values = Record;
           Listener->SetBID();
         }
         break;
     }
   }
 }