[llvm] [utils] update_llc_test_checks --downstream-target-handler-path option (PR #135879)

[Reid Kleckner via llvm-commits]

rnk wrote:

I think LLVM knows that downstream out-of-tree targets exist, and we should be willing to take reasonable, low-cost steps to make them easier to maintain. The entire TargetMachine backend interface is virtual, so we did 80% of the work to make LLVM support out-of-tree plugins as loadable modules, but then we stopped at some point and started adding Triple ISA checks throughout the compiler, compromising the abstraction. I think we ought to lean in the direction of minimizing target awareness and increasing abstraction over all targets, and that's the intuition I'd use the judge this. If it handles out of tree targets uniformly with our in-tree set, then it's a reasonable cost for us to bear.

https://github.com/llvm/llvm-project/pull/135879