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

[Nico Weber via llvm-commits]

nico wrote:

If we do this, what's the advantage of putting all these headers in Config/? `LLVM_ENABLE_DIA_SDK` for example is used exclusively in llvm/tools/llvm-symbolizer/llvm-symbolizer.cpp – should we put the header for that over there then?

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I think this is a noble patch, but doing this piecemeal for the ones people actually toggle is imho good enough. (I _would_ love to be able to turn assertions on and off without a full rebuild, but even if that's in its own .h, assertions are used anywhere, so it's still a full rebuild even then :P)

And putting the archs header over in mlir so that we can't end up with code that has ifdef checks for archs also seems nice from a a "keep llvm code cleaner" PoV.

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