Revert "Revert "Update aosp/master libcxx rebase to r263688""

This reverts commit 1d4a1edbc7.

Change-Id: I2909937fe582f2c5552bc86e7f4d2d5cff0de0aa

test/libcxx/atomics/libcpp-has-no-threads.pass.cpp

@@ -0,0 +1,18 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// XFAIL: libcpp-has-no-threads
+
+#ifdef _LIBCPP_HAS_NO_THREADS
+#error This should be XFAIL'd for the purpose of detecting that the LIT feature\
+   'libcpp-has-no-threads' is available iff _LIBCPP_HAS_NO_THREADS is defined
+#endif
+
+int main()
+{
+}