Use llvmPackages_7 explicitly for ccls

[Kaali]

llvmPackages_latest picks too old LLVM for ccls on macOS. Which breaks
the build.

Motivation for this change

For some reason after merging #56673 llvmPackages_latest behaviour has changed. Instead of using the actual latest version on macOS, it uses a version that has Clang 5.x which is too old for ccls.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

[Mic92]

Out of interest, what does

$ nix eval '(with import <nixpkgs> {}; llvmPackages_latest.llvm.name)'

return on your machine?

[Mic92]

This should be llvm_8 right now, so this is a downgrade to llvm_7.

[Mic92]

So maybe the actual problem is, that it does not work with llvm 8.

[Kaali]

Ah. That might actually be it. Maybe the version 5 thing was me misreading build output.

[Kaali]

Out of interest, what does

$ nix eval '(with import <nixpkgs> {}; llvmPackages_latest.llvm.name)'

return on your machine?

"llvm-8.0.0rc2" so it actually finds 8. So I guess you are correct above that the problem is with 8. So forcing to 7 is accidentally the right solution to this I guess.