message: Make a static const variable constexpr in BitSetIterator
  This was found during some changes in Google3.
  Also renames BitsPerWord to kBitsPerWord.
diff: @@ -51,7 +51,7 @@ class BitSetIterator final {
  private:
  unsigned long getNextBit();
  - static const size_t BitsPerWord = sizeof(uint32_t) * 8;
  + static constexpr size_t kBitsPerWord = sizeof(uint32_t) * 8;
  std::bitset<N> mBits;
  unsigned long mCurrentBit;
  unsigned long mOffset;
  @@ -88,7 +88,7 @@ BitSetIterator<N, T>::Iterator::Iterator(const std::bitset<N>& bits)
  if (bits.any()) {
  mCurrentBit = getNextBit();
  } else {
  - mOffset = static_cast<unsigned long>(roundUp(N, BitsPerWord));
  + mOffset = static_cast<unsigned long>(roundUp(N, kBitsPerWord));
  }
  }
  @@ -120,8 +120,8 @@ unsigned long BitSetIterator<N, T>::Iterator::getNextBit() {
  return ScanForward(wordBits) + mOffset;
  }
  - mBits >>= BitsPerWord;
  - mOffset += BitsPerWord;
  + mBits >>= kBitsPerWord;
  + mOffset += kBitsPerWord;
  }
  return 0;
  }
  