[llvm] [NFC] clean up memtag-stack code (PR #80906)

[Florian Mayer via llvm-commits]

fmayer wrote:

> Essentially the path taken in the _old_ code: the debug records get RAUW'd to `TagPCall`, but then later they get replaced back to the "original" `Info.AI`?

Not the very original one because it was TrackingVH, so it was the `Info.AI` _after_ the align and pad call. AFAICT the code was like this by historical accident.

https://github.com/llvm/llvm-project/pull/80906