Silence GCC warning about dropping off a fully covered switch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194077 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Bitcode/Reader/BitcodeReader.cpp

@@ -3273,6 +3273,7 @@ class BitcodeErrorCategoryType : public _do_message {
     case BitcodeReader::InvalidValue:
       return "Invalid value";
     }
+    llvm_unreachable("Unknown error type!");
   }
 };
 }