[libc++] Add the version header to all headers.

Some headers which require the version header depend on other headers to
provide it. Include the version header in all top-level headers to make
sure a header cleanup can't remove the version header.

Note this doesn't add the version header to the c headers.

Reviewed By: #libc, Quuxplusone, ldionne

Differential Revision: https://reviews.llvm.org/D116172

NOKEYCHECK=True
GitOrigin-RevId: bd6e6846e7f431e9d84b1f257d4a0c8ddeac4fe2
diff --git a/include/valarray b/include/valarray
index 909e042..a55d921 100644
--- a/include/valarray
+++ b/include/valarray
@@ -348,6 +348,7 @@
 #include <functional>
 #include <initializer_list>
 #include <new>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header