Remove compiler warning when building.

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 88 matching lines @@
     (void)ReadVBR64((unsigned)Op.getValue());
     break;
   case NaClBitCodeAbbrevOp::Array:
     // This can't happen because the abbreviation must be valid.
     llvm_unreachable("Bad array abbreviation encoding!");
     break;
   case NaClBitCodeAbbrevOp::Char6:
     (void)Read(6);
     break;
   }
+  llvm_unreachable("Illegal abbreviation encoding for field!");
 }
 
 /// skipRecord - Read the current record and discard it.
 void NaClBitstreamCursor::skipRecord(unsigned AbbrevID) {
   // Skip unabbreviated records by reading past their entries.
   if (AbbrevID == naclbitc::UNABBREV_RECORD) {
     unsigned Code = ReadVBR(6);
     (void)Code;
     unsigned NumElts = ReadVBR(6);
     for (unsigned i = 0; i != NumElts; ++i)
@@ skipping 316 matching lines @@
         UpdateAbbrevs =
             &BitStream->getBlockInfo((unsigned)Record[0])->getAbbrevs();
         if (Listener) {
           Listener->Values = Record;
           Listener->SetBID();
         }
         break;
     }
   }
 }