Libc++'s policy is that we only officially support configurations that
we have CI for. This has been instrumental in providing an implementation
of quality while ensuring low friction for contributors.
We've been making an exception for FreeBSD because it has been supported
for a long time and it's an important platform. However, after requesting
FreeBSD CI for more than a year, it is time to stop making an exception
and hold all platforms to the same standards.
This will be committed after branching for LLVM 15 unless FreeBSD CI is
added before then.