[libc++][CI] Adds GCC trunk image. (#132271)
As discussed during the last monthly meeting we want to be able to test
the GCC development version in our CI, but we don't want to commit to
support this compiler version.
This adds the image to the CI Docker image. At the moment GCC-15 will
not pass the CI. Having it in the Docker image makes it possible to
validate fixes.
---------
Co-authored-by: Louis Dionne <ldionne.2@gmail.com>
Co-authored-by: Hristo Hristov <hghristov.rmm@gmail.com>
