Replace documentation mentions of IRC with Discord (#114276)

This does not touch code owners or credits files that list IRC handles,
that can be done separately if we want to make that change.

See
https://discourse.llvm.org/t/rfc-remove-irc-as-a-recommended-communication-channel/82808/3
for the RFC.

GitOrigin-RevId: 0ab44fd2464354dfdca0e7afacbb21a84bca46d9