[llvm] [DWARFLinker] Simplify clang module unit bookkeeping (NFC) (PR #217758)

[Jonas Devlieghere via llvm-commits]

JDevlieghere wrote:

> OMG. I thought I found a typo, but this method is literally called `getContaingFile()` [sic!]?

:D

https://github.com/llvm/llvm-project/pull/217957

https://github.com/llvm/llvm-project/pull/217758