[llvm] [mlir] Shard the public llvm-config.h in multiple files (NFC) (PR #71273)

[Chris B via llvm-commits]

llvm-beanz wrote:

@joker-eph I think the important point here is that you need to justify why this is an improvement. Do you have performance numbers to show that this speeds up builds in some configuration? We can argue about subjectives all day and night, but if you have data to justify making this change it completely shifts the discussion.

https://github.com/llvm/llvm-project/pull/71273