Update C++ style guide.
Update C++ style guide.

- Reword guidance for definitions of inline functions and templates.
- Capitalize the first letter of various comments.
- Update command-line flag reference to use absl flags.
- Improve braced-initialization example for 64-bit constants.
- Delete C++03 guidance for NULL.
- Clarify wording around wrapping of string literals.
- Use C++17 [[fallthrough]] instead of ABSL_FALLTHROUGH.

google/styleguide,google/styleguide,google/styleguide,google/styleguide