Implement the infrastructure for feature-test macros. Very few actual feature test macros, though. Reviewed as: https://reviews.llvm.org/D51955

llvm-svn: 342073
diff --git a/libcxx/include/memory b/libcxx/include/memory
index de3f52d..276b3b7 100644
--- a/libcxx/include/memory
+++ b/libcxx/include/memory
@@ -667,6 +667,7 @@
 #if !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
 #  include <atomic>
 #endif
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header