Emit an error when include <atomic> after <stdatomic.h>

C11 defines `kill_dependency` as a macro in <stdatomic.h>. When you
include <atomic> after <stdatomic.h>, the macro clashes with
`std::kill_dependency` and causes multiple errors. Explicit error should
help in diagnosing those errors.

No change for working code that includes <atomic> before <stdatomic.h>.

rdar://problem/27435938

Reviewers: rsmith, EricWF, mclow.lists, jfb

Reviewed By: jfb

Subscribers: jfb, jkorous-apple, christof, bumblebritches57, JonChesterfield, smeenai, cfe-commits

Differential Revision: https://reviews.llvm.org/D45470



git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@332413 91177308-0d34-0410-b5e6-96231b3b80d8

include/atomic

@@ -555,6 +555,9 @@ void atomic_signal_fence(memory_order m) noexcept;
 #if !defined(_LIBCPP_HAS_C_ATOMIC_IMP) && !defined(_LIBCPP_HAS_GCC_ATOMIC_IMP)
 #error <atomic> is not implemented
 #endif
+#ifdef kill_dependency
+#error C++ standard library is incompatible with <stdatomic.h>
+#endif
 
 #if _LIBCPP_STD_VER > 14
 # define __cpp_lib_atomic_is_always_lock_free 201603L