Since LLVM 17 has been branched and is on the verge of being released,
we can drop the CI job that tests against Clang 15. I think the number
of cherry-picks to `release/17.x` will be a lot smaller now, so keeping
a Clang 15 job around for that purpose seems unnecessary.
As a fly-by, this patch also removes some Clang 15 workarounds and test
suite annotations as we usually do. It also removes some slightly older
gcc test suite annotations that were missed.
AppleClang 15 was released on September 18th and is now stable. Per our
policy, we're bumping the supported AppleClang compiler to the latest
release. This allows cleaning up the test suite, but most importantly
unblocking various other patches that are blocked on bumping the
compiler requirements.
