Fix nodiscard test when modules are enabled

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@318618 91177308-0d34-0410-b5e6-96231b3b80d8

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>