fmtlib: backport upstream fixes to suppress bogus gcc 13.1 warnings

gcc 13.1 erroneously warns of dangling references when using our custom
formatters. This was reported to both fmtlib and gcc and fixes have been
provided, but are not released yet.

This change backports two fixes from the master branch to our vendored
version:
https://github.com/fmtlib/fmt/commit/f61f15cc5b11582d50d02ba0514c5344f7b2600e
https://github.com/fmtlib/fmt/commit/ef55d4f52ec527668a8e910a56ea79d9b939dbc2

For more information on the issue, see:
https://github.com/fmtlib/fmt/issues/3415
https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;h=6b927b1297e66e26e62e722bf15c921dcbbd25b9

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I30bbbbe5e0aa2729e50228acdb528ee060d9df23