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/array b/include/array
index ba4ff42..8f4e111 100644
--- a/include/array
+++ b/include/array
@@ -112,6 +112,7 @@
 #include <algorithm>
 #include <stdexcept>
 #include <cstdlib> // for _LIBCPP_UNREACHABLE
+#include <version>
 #include <__debug>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)