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

[Joshua Cranmer via cfe-commits]

https://github.com/jcranmer-intel commented:

One thing I have noticed is that this does seem to more or less by accident remove the IRC channel from the list of communication channels to which the code of conduct and other policies applied.

I am on board with generally replacing use of IRC with Discord, and preferring discord especially in our documentation, but I wonder if it might make sense to have some residual mention somewhere that #llvm on the OFTC network as being project-affiliated that could cover that case (even if we emphasize that it's not particularly active). I don't have any good wording suggestions, I'm afraid.

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