Revert "Update aosp/master libcxx rebase to r263688"

The world is burning.

This reverts commit c004fd909c, reversing
changes made to 1418e4163d.

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

@@ -1,18 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-//                     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()
-{
-}