Replace `ABSL_ATTRIBUTE_UNUSED`:

* With commented-out parameter name if it applies to a function parameter which
  is never used. This is more idiomatic and shorter.

* With `[[maybe_unused]]` otherwise, i.e. the parameter is used conditionally
  (due to `if constexpr`, possibly empty variadic expansion, or `#if`) or the
  name is something other than a function parameter.

The parameter name is still skipped altogether when the parameter is never used
to pass a value but e.g. disambiguates overloads (`operator++(int)`,
`std::in_place_t`), specifies a type where an explicit template parameter
would not work, or introduces template parameters through
`std::index_sequence<indices...>`.

Replace `ABSL_ATTRIBUTE_NORETURN` with `[[noreturn]]`. It is available since
C++11.

Replace `ABSL_FALLTHROUGH_INTENDED` with `[[fallthrough]]`. It is available
since C++17.

Attribute macros for non-standard and thus non-portable attributes remain.

PiperOrigin-RevId: 912676701