Use report_fatal_error instead of llvm_unreachable, so we don't crash on user input

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226248 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Bitcode/BitCodes.h

@@ -125,7 +125,7 @@ public:
     case Blob:
       return false;
     }
-    llvm_unreachable("Invalid encoding");
+    report_fatal_error("Invalid encoding");
   }
 
   /// isChar6 - Return true if this character is legal in the Char6 encoding.