In addition to the circular dependency, your test plan is not sufficient. As the kernel maintainer, I have no interest in Clang builds at the moment, so that's not a valid test. As the mesalib maintainer, I will not accept this patch until you have rebuilt and tested mesalib against this. Given that only a few other packages directly link against LLVM and the number of affected users if they are broken, I also expect that you test them as well:
When trying to build beignet it complains quite a bit. I tried the llvm9 patch from FreeBSD but still has some issues. I'm not informed enough to fix most issues with 9.0, so should I abandon it or leave it here in case someone else wants to continue on it?
I hate to step on your toes but i have had this upgrade done locally, with rebuilds done and tested, for a couple of weeks now. The only thing i've been waiting on is a new release of python-llvmlite which is why it hasn't been pushed yet (but, it looks like llvmlite can just be patched, as the issues are windows localized).