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

pkgs/development/compilers/rust/rustc.nix

@@ -155,8 +155,10 @@ in stdenv.mkDerivation rec {
 
   requiredSystemFeatures = [ "big-parallel" ];
 
+  passthru.llvm = llvmShared;
+
   meta = with stdenv.lib; {
-    homepage = https://www.rust-lang.org/;
+    homepage = "https://www.rust-lang.org/";
     description = "A safe, concurrent, practical language";
     maintainers = with maintainers; [ madjar cstrahan globin havvy ];
     license = [ licenses.mit licenses.asl20 ];