<bit> produces warnings when building with C++17/13/11/... (#1455)

Summary:
Constant from: http://eel.is/c++draft/cpp.predefined#1.1

Fixes https://github.com/facebook/folly/issues/1454

Pull Request resolved: https://github.com/facebook/folly/pull/1455

Reviewed By: yfeldblum

Differential Revision: D23702274

Pulled By: Orvid

fbshipit-source-id: a08f922cc11c086362b91c93922b59c9a5c62e51

folly/lang/Bits.h

@@ -67,7 +67,7 @@
 #include <folly/lang/Assume.h>
 #include <folly/portability/Builtins.h>
 
-#if __has_include(<bit>)
+#if __has_include(<bit>) && __cplusplus >= 202002L
 #include <bit>
 #endif