Reject attempts to build a module without -fmodules, rather than silently doing weird things.

llvm-svn: 307316
diff --git a/clang/test/Modules/preprocess-build.cpp b/clang/test/Modules/preprocess-build.cpp
index bf9f161..36a0740 100644
--- a/clang/test/Modules/preprocess-build.cpp
+++ b/clang/test/Modules/preprocess-build.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -std=c++1z %s -verify
+// RUN: %clang_cc1 -std=c++1z -fmodules %s -verify
 
 #pragma clang module build baz
   module baz {}