[llvm] [DebugInfo] Update DebugInfoFinder to take retainedNodes into account (PR #140285)

[Vladislav Dzhidzhoev via llvm-commits]

dzhidzhoev wrote:

> Change LGTM, but might be a good idea to add a DIImportedEntity to the test as well.

The test case was added. Should I merge it? (Asking since the "approve" button wasn't pressed 🙂 ).

https://github.com/llvm/llvm-project/pull/140285