Do all the major package managers do this or is it just apt? I have a relatively new version of Julia from pacman on Arch, I wonder if it has a patched LLVM or not...
They don't require you to use “Linux's LLVM” (which would not make any sense given that Linux and LLVM are two independent projects), they make you use the version of LLVM they are currently packaging.
> Do all the major package managers do this or is it just apt?
All the package managers will conceptually do that; however, how old is the considered LLVM will vary strongly depending on the packaging policy of your distribution.