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
diff --git a/lib/Bitcode/Reader/BitcodeReader.cpp b/lib/Bitcode/Reader/BitcodeReader.cpp
index d571fd8..f2039d4 100644
--- a/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -3273,6 +3273,7 @@
     case BitcodeReader::InvalidValue:
       return "Invalid value";
     }
+    llvm_unreachable("Unknown error type!");
   }
 };
 }