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

[Petr Hosek via llvm-commits]

petrhosek wrote:

`LLVM_VERSION_*` changes every 6 months, I don't think that's something we should be optimizing for (but I'd be still fine moving it to `llvm-version.h` if you think it's worth it). For all other options like `LLVM_ENABLE_CURL`, to change the value you need to reconfigure the build, is that a scenario that developers do often enough that we need to optimize for?

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