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

[Nico Weber via llvm-commits]

nico wrote:

> Is this a common thing to switch back and forth?

I do it several times a week at least. I usually only build with host arch, and if I need to enable an additional arch to debug a per-arch thing I turn on that arch. That used to be very quick, and now it isn't. (As you say, it's a full rebuild without any cache hits.)

Maybe it could be a generated header in mlir for gpu targets only? Then it wouldn't affect me at all. Putting it in llvm-config.h just because the file already exists seems a bit weird – half of llvm-config.h are these defines now.

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