[llvm] [SelectionDAG] Fix null pointer dereference in resolveDanglingDebugInfo (PR #173947)

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MetalOxideSemi wrote:

@arsenm Hi, I managed to borrow a macOS device and it turned out to have nothing to do with host system. Targeting apple-darwin will always crash no matter which host it is running on, which matches your observation on the failure of the previous PR is due to using default target (hence running on a macOS device causes it to targeting macOS and crash). I've fixed llvm-lit instruction and added XFAIL on darwin targets. Thanks for your patience!

https://github.com/llvm/llvm-project/pull/173947