[llvm] [CMake] Add tablegen job pool support (PR #84762)

[Nicolai Hähnle via llvm-commits]

https://github.com/nhaehnle approved this pull request.

Thanks! I did run into the problem of builds aborting due to tablegen out of memory in the past, so this makes sense to me.

It looks reasonable to me, but I'm not a CMake expert, so perhaps give it a bit for somebody else to chime in.

https://github.com/llvm/llvm-project/pull/84762