[clang] [libunwind] [llvm] Replace documentation mentions of IRC with Discord (PR #114276)

[Kristof Beyls via cfe-commits]

https://github.com/kbeyls commented:

Overall looks great, thank you :)
I've left one nit-picky comment on keeping IRC mentioned in the code of conduct page, as even though it's a "legacy" communication channel, it continues to exist, so the code-of-conduct continues to apply there.

https://github.com/llvm/llvm-project/pull/114276