Mk/bsd.default-versions.mk: Update llvm to newer version

Hello all,

I have some doubts about the procedure to updating default llvm to a newest
version and how can I help testing a new default version.

Right now we have LLVM_DEFAULT?=15 and next possible versions could be 16,
17 or 18. What version should I start testing?

One of my motivations is to reduce builds dependency to just one llvm
instead of two: 15 and 17 happening on my ports list build.

Well, as we see, I really need some enlightment in this subject :)

Thanks,

-- 
Nuno Teixeira
FreeBSD UNIX:  <eduardo@FreeBSD.org>   Web:  https://FreeBSD.org