The commit cd5808eb introduced a union as a storage for the format
argument passed to the internal API fmt::detail::make_arg. This was done
to solve the issue that the internal API no longer accepted temporary
variables.

However, it's generally better to avoid using internal APIs, as they are
more likely to have breaking changes in the future. Instead, we can use
the public API fmt::dynamic_format_arg_store to dynamically build the
argument list. This API accepts temporary variables, and its behavior is
more stable than the internal API. `libfmt.cmake` is updated to reflect
the change as well.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.