Eliminate "enumeration value not handled in switch" warnings

(Caused by r240370)

llvm-svn: 240376
diff --git a/clang/lib/Driver/ToolChains.cpp b/clang/lib/Driver/ToolChains.cpp
index b2e8051..6ec855b 100644
--- a/clang/lib/Driver/ToolChains.cpp
+++ b/clang/lib/Driver/ToolChains.cpp
@@ -2455,6 +2455,8 @@
     llvm::sys::path::append(P, "x86_64-nacl/include/c++/v1");
     addSystemInclude(DriverArgs, CC1Args, P.str());
     break;
+  default:
+    break;
   }
 }