CL https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/1351315 updated llvm/clang SLOT from 0/8.0 to 8, to match what Gentoo does.

When toolchain team decide to update to LLVM 9, they'll need to figure out:
 1. https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/1351315/3#message-0bed28f7fbdd8ebf9b92b82dbe9c65b78f4f962c
"drinkcat> Yes, but how are we going to migrate? I assume we wouldn't want both llvm 8 and 9 installed?
vapier> we can update our tooling to manage that.  we had the same problem with gcc where every version was a new SLOT.  prob want to file a bug to track, and the toolchain team can worry about it when llvm 9 shows up ;).

although they've already been thinking about supporting multiple SLOTs of llvm, so maybe this is just a step in that direction.
"
 2. Would be best to avoid issue seen in https://bugs.chromium.org/p/chromium/issues/detail?id=910430, where we had to wait for prebuilts to be regenerated (possibly LLVM 8 to 9 won't have this issue, as files from 8 and 9 should not conflict?)