Fix the check for #include <bit>

Previously this was guarded with macros that are defined by <bit> itself.

Note that libc++ also had a bug that was fixed last week
https://github.com/llvm/llvm-project/commit/a4f0764aefd6ea41e83a5974f582a1a7e985667d

PiperOrigin-RevId: 543511181
Change-Id: I1b8efa32f721ad450f8d1803c6c6c8373ad0371c

absl/numeric/bits.h

@@ -38,13 +38,13 @@
 #include <limits>
 #include <type_traits>
 
-#if (defined(__cpp_lib_int_pow2) && __cpp_lib_int_pow2 >= 202002L) || \
-    (defined(__cpp_lib_bitops) && __cpp_lib_bitops >= 201907L)
+#include "absl/base/config.h"
+
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
 #include <bit>
 #endif
 
 #include "absl/base/attributes.h"
-#include "absl/base/config.h"
 #include "absl/numeric/internal/bits.h"
 
 namespace absl {