chore(3p/overlays): Remove LLVM11 bump from overlay r/2622

* This was mostly for //third_party/nix and its dependencies which now
  have been set to use llvmPackages_11 manually.

* For //users/grfn/achilles we also manually select the newer LLVM version.

* //tools/cheddar doesn't seem to need llvm anymore.

* //third_party/buzz also compiles with clang 7.1.0

* replace clang-tools everywhere with new attribute clang-tools_11

For the future we may want to have something similar again, but it may
not be necessary to invest too much time into it: nixpkgs is set to
upgrade their default llvmPackages to LLVM 11 as well at some point in
the near future.

Co-Authored-By: sterni <sternenseemann@systemli.org>
Change-Id: Id83868dbc476a6c776b59518b856c933f30ea79d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3135
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: grfn <grfn@gws.fyi>

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