Remove test_atomic.h header

Because <atomic> can now be used in C++03 there is no need for the test_atomic.h header.
This commit removes the header and converts all usages to use <atomic> instead.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@245468 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/thread/thread.threads/thread.thread.class/thread.thread.member/detach.pass.cpp

@@ -16,11 +16,10 @@
 // void detach();
 
 #include <thread>
+#include <atomic>
 #include <cassert>
 
-#include "test_atomic.h"
-
-AtomicBool done(false);
+std::atomic_bool done(false);
 
 class G
 {