[llvm] [VPlan] Use Unknown instead of empty location in VPlanTransforms (PR #157702)

[Florian Hahn via llvm-commits]

fhahn wrote:

@SLTozer is there any way to avoid/get notified when new uses of the default constructor are added? would be nice if we could avoid backsliding

https://github.com/llvm/llvm-project/pull/157702