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

[Mehdi Amini via llvm-commits]

joker-eph wrote:

 Thinking a bit more about this: seems to me that the underlying problem with respect to caching and rebuilding is that  the llvm-config.h file is monolithic and every flag adds to the burden. 
What about splitting it? We can have a subfolder `configs/xxx.h` for every flag (or grouping of flags) and the top-level llvm-config.h including all of them.
Then it's a matter of migrating current includes of `llvm-config.h` to include the specific `configs/xxx.h`, this should provide the finer grain rebuild we could expect?

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