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/bit b/libcxx/include/bit
index 5bf5214..db3812e 100644
--- a/libcxx/include/bit
+++ b/libcxx/include/bit
@@ -21,6 +21,7 @@
 */
 
 #include <__config>
+#include <version>
 
 #if defined(__IBMCPP__)
 #include "support/ibm/support.h"