Remove unreachable code in Clang. (replace with llvm_unreachable where appropriate or when GCC requires it)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@148292 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Basic/IdentifierTable.cpp b/lib/Basic/IdentifierTable.cpp
index c191456..a76a363 100644
--- a/lib/Basic/IdentifierTable.cpp
+++ b/lib/Basic/IdentifierTable.cpp
@@ -19,6 +19,7 @@
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/Support/ErrorHandling.h"
 #include <cstdio>
 
 using namespace clang;
@@ -503,5 +504,5 @@
 #include "clang/Basic/OperatorKinds.def"
   }
 
-  return 0;
+  llvm_unreachable("Invalid OverloadedOperatorKind!");
 }