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

[Stephen Tozer via llvm-commits]

SLTozer 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

We don't have an automated notification at the moment, but the buildbot here is now green: https://lab.llvm.org/staging/#/builders/222

Any new unannotated locations should cause a failure on the buildbot (if they occur on the buildbot, which is running an `-O2 -g` build of CTMark); since the buildbot is on staging it won't automatically email patch authors that break the bot, but the output on failure includes stack traces that should make the cause of the error quite explicit ([example](https://lab.llvm.org/staging/#/builders/222/builds/300/steps/12/logs/stdio)), so it should be sufficient to check if every now and then to see that it is still green - though also, I'll be doing the same and manually commenting on merged PRs that introduce new missing annotations.

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