[ASTImporter] Removed uneeded default case label.

llvm-svn: 344784
diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index 200214b..ec345bb 100644
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -89,9 +89,9 @@
       return "UnsupportedConstruct";
     case Unknown:
       return "Unknown error";
-    default:
-      llvm_unreachable("Invalid error code.");
     }
+    llvm_unreachable("Invalid error code.");
+    return "Invalid error code.";
   }
 
   void ImportError::log(raw_ostream &OS) const {