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

[Mehdi Amini via llvm-commits]

joker-eph wrote:

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

Something fishy: this change shouldn't change anything until we update users.

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