Add 'llvm_unreachable' to passify GCC's understanding of the constraints

of several newly un-defaulted switches. This also helps optimizers
(including LLVM's) recognize that every case is covered, and we should
assume as much.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147861 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Bitcode/BitCodes.h

@@ -20,6 +20,7 @@
 
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/Support/DataTypes.h"
+#include "llvm/Support/ErrorHandling.h"
 #include <cassert>
 
 namespace llvm {
@@ -122,6 +123,7 @@ public:
     case Blob:
       return false;
     }
+    llvm_unreachable("Invalid encoding");
   }
 
   /// isChar6 - Return true if this character is legal in the Char6 encoding.