[llvm] [DLCov][NFC] Propagate annotated DebugLocs through transformations (PR #138047)

[Orlando Cazalet-Hyams via llvm-commits]

https://github.com/OCHyams commented:

> Mostly LGTM with a couple of double-check questions inline. I wonder if there's anything we can do to avoid future usage of DILocation::getMergedLocation. Could private + friend work here? Off the top of my head I'm not sure if anything else other than the new DebugLoc::getMergedLocation needs to call it?

I think this comment still stands but otherwise LGTM

https://github.com/llvm/llvm-project/pull/138047