[libc++] Mark lock_guard nodiscard test as unsupported in C++03

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@369672 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/thread/thread.lock/thread.lock.guard/nodiscard.fail.cpp

diff --git a/test/libcxx/thread/thread.lock/thread.lock.guard/nodiscard.fail.cpp b/test/libcxx/thread/thread.lock/thread.lock.guard/nodiscard.fail.cpp
index 67fbc54..ab7c7eb 100644
--- a/test/libcxx/thread/thread.lock/thread.lock.guard/nodiscard.fail.cpp
+++ b/test/libcxx/thread/thread.lock/thread.lock.guard/nodiscard.fail.cpp
@@ -12,6 +12,9 @@
 // UNSUPPORTED: clang-6, clang-7, clang-8, clang-9
 // UNSUPPORTED: apple-clang-9, apple-clang-10, apple-clang-11
 
+// [[nodiscard]] isn't supported in C++98 and C++03 (not even as an extension)
+// UNSUPPORTED: c++98, c++03
+
 // <mutex>
 
 // template <class Mutex> class lock_guard;