[llvm] [DebugInfo] Add Verifier check for incorrectly-scoped retainedNodes (PR #166855)

[Jeremy Morse via llvm-commits]

jmorse wrote:

Very good; it's worth putting that in the commit message, so that future code-archeologists discovering strange test patterns can work out that it was manual editing that shaped the IR inputs, not a version of LLVM that they need to support.

https://github.com/llvm/llvm-project/pull/166855