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

[Fangrui Song via llvm-commits]

MaskRay wrote:

I prefer not to engage in the debate and hope we can focus on the technical aspects instead.

Splitting a config header into multiple files strikes me as somewhat unusual.

I don't frequently update `LLVM_TARGETS_TO_BUILD`, but I do occasionally update `LLVM_EXPERIMENTAL_TARGETS_TO_BUILD`.
I typically build all experimental targets to avoid unnecessarily causing issues with my changes.
However, some experimental targets might encounter random failures, prompting me me to remove them from the list. (Sometimes I try to fix them instead).
I'll be happy if I don't need to rebuild all targets if I remove a single entry from `LLVM_EXPERIMENTAL_TARGETS_TO_BUILD`.

As for other configurations like curl,zstd,threads,etc, I rarely make updates, and I am okay with the rebuild time if I do decide to change them.


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