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

[Fabian Mora via llvm-commits]

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

LGTM! And I can confirm that enabling or disabling targets has a minimal rebuild impact if Ccache is enabled.

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