llvmPackages_11: start with 11.0.0rc2

[ggreif]

http://lists.llvm.org/pipermail/release-testers/2020-August/001321.html
http://lists.llvm.org/pipermail/release-testers/2020-July/001305.html

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[ggreif]

@GrahamcOfBorg build lldb_11

[ggreif]

Yes, removing .patch files next.

[ggreif]

I get

CMake Error at CMakeLists.txt:4 (message):
  libc++abi now requires being built in a monorepo layout with libcxx
  available

On macOS.

[primeos]

Any news on this? Would be good to have this soon for testing chromiumBeta (though I could also apply this PR locally).

Is enablePolly = true the only problem that's left?

Also cc @lovek323 @7c6f434c @dtzWill (LLVM maintainers)

[ggreif]

Unfortunately the macOS build fails with #94204 (comment) and I have no time right now to fix it. Probably easy to build a monorepo-like layout from the downloaded tarballs, and I assume that not even all are needed. But I have no time right now to do the grunt work, so we can either merge this and accept that Darwin is broken, or start working on it. Polly is well out of my reach, sorry.

[primeos]

Thanks for the update. Given that this is a new package I would be fine with getting the Darwin support in place later if we have to.

@NixOS/darwin-maintainers do you maybe have time to take a look at the Darwin build?

[ggreif]

@GrahamcOfBorg build clang_11

[primeos]

Ok, so I just refactored/squashed the history and merged this in 28aef45 :)

Thanks for your work @ggreif and @conferno ❤️