Fix a default on covered switch found by -Wcovered-switch-default

Introduced in r178207

llvm-svn: 178212
diff --git a/clang-tools-extra/modularize/Modularize.cpp b/clang-tools-extra/modularize/Modularize.cpp
index c30bff7..9b6e656 100644
--- a/clang-tools-extra/modularize/Modularize.cpp
+++ b/clang-tools-extra/modularize/Modularize.cpp
@@ -233,9 +233,8 @@
   case Macro:
     return "macro";
     break;
-  default:
-    return "unknown";
   }
+  llvm_unreachable("invalid Entry kind");
 }
 
 struct HeaderEntry {