Include <bit> in folly/lang/Bits.h (#1334)

Summary:
[Folly] Add `#include <bit>` in `folly/lang/Bits.h` when that header, which is introduced in C++20, is available.

Fixes https://github.com/facebook/folly/issues/1333.
Closes https://github.com/facebook/folly/pull/1334.

Reviewed By: luciang, markisaa

Differential Revision: D20369239

fbshipit-source-id: fee8682c23107a1da17c35abf5a9e16bb5177cdd

folly/lang/Bits.h

@@ -64,6 +64,10 @@
 #include <folly/lang/Assume.h>
 #include <folly/portability/Builtins.h>
 
+#if __has_include(<bit>)
+#include <bit>
+#endif
+
 namespace folly {
 
 #if __cpp_lib_bit_cast