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

[Fabian Mora via llvm-commits]

fabianmcg wrote:

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

Switching on & off the target does prompt ninja to rebuild everything, however, if a normal build takes in my machine 10 min, the rebuild of all targets took 2 min with Ccache.

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