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

llvm-svn: 342073
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: f56972e22489c92028e5147344ab8b31b0b4fed5
diff --git a/include/tuple b/include/tuple
index b3a17e7..fb5428e 100644
--- a/include/tuple
+++ b/include/tuple
@@ -141,6 +141,7 @@
 #include <type_traits>
 #include <__functional_base>
 #include <utility>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header