Add LLVM 15 (Vanilla) CI Job

[thewilsonator]

This is expected to fail for the time being, but we're almost there!

fixes #4042

[thewilsonator]

curl -fL --retry 3 --max-time 300 -o llvm.tar.xz https://github.com/llvm/llvm-project/releases/download/llvmorg-15.0.0/clang+llvm-15.0.0-x86_64-linux-gnu-ubuntu-18.04.tar.xz

gives 404, and https://github.com/llvm/llvm-project/releases/tag/llvmorg-15.0.0 indeed seems to not have that combination, neither does 15.0.1.

[GavinRay97]

The last few LLVM releases don't seem to have an ubuntu target built, but some oddball architectures like PowerPC and MIPS are present. It's a bit strange.

If you happen to find out why, I'd love to know too.

[thewilsonator]

superseded by #4257