Fix nodiscard test when modules are enabled

llvm-svn: 318618
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 518f24b5b9478f0680e0c30db8835c5a15f3096a
diff --git a/test/libcxx/diagnostics/nodiscard.pass.cpp b/test/libcxx/diagnostics/nodiscard.pass.cpp
index 7519799..d308248 100644
--- a/test/libcxx/diagnostics/nodiscard.pass.cpp
+++ b/test/libcxx/diagnostics/nodiscard.pass.cpp
@@ -13,6 +13,7 @@
 
 // UNSUPPORTED: c++98, c++03, c++11, c++14
 
+// MODULES_DEFINES: _LIBCPP_DISABLE_NODISCARD_AFTER_CXX17
 #define _LIBCPP_DISABLE_NODISCARD_AFTER_CXX17
 #include <__config>