[llvm] Improve llvm-config.h to record which target is configured or not (PR #71164)

[Fabian Mora via llvm-commits]

fabianmcg wrote:

You're right, I was just proposing avoiding a bigger refactor as splitting it means going through each include of `llvm-config.h` and only including the necessary files. I think `Targets.h`, `Version.h`, `TargetDefault.h`, `BuildOptions.h` would do it.

https://github.com/llvm/llvm-project/pull/71164