use log2p1 instead of bit_width

Author: jonkeane

cpp/src/arrow/util/bit_util.h

@@ -138,7 +138,14 @@ static inline uint64_t TrailingBits(uint64_t v, int num_bits) {
 // Returns ceil(log2(x)).
 static inline int Log2(uint64_t x) {
   // DCHECK_GT(x, 0);
+
+  // TODO: We can remove this condition once CRAN upgrades its macOS
+  // SDK from 11.3.
+#if defined(__clang__) && !defined(__cpp_lib_bitops)
+  return std::log2p1(x - 1);
+#else
   return std::bit_width(x - 1);
+#endif
 }
 
 //