clippy: add rustc.llvm to buildInputs

The Hydra build [1] failed because it was unable to link to `LLVM9`; add
`llvmShared` to `passthru` in order to stay up to date with required
LLVM versions. Also quote the homepage URLs, since that's preferred.

[1] https://hydra.nixos.org/build/112989779/nixlog/1