[clang] [clang] NFCI: Use `FileEntryRef` in `SourceManager::FileInfos` (PR #67742)

[Ben Langmuir via cfe-commits]

https://github.com/benlangmuir commented:

I'm skeptical that treating `FileEntryRef` as `FileEntry *` for the purposes of equality and hashing is a good idea.  To me, the obvious interpretation of `FileEntryRef ==` is that the path must be equal, and having it based on the dev/inode is surprising.  I know that's not something you're *introducing* in this PR, but I would prefer we remove that functionality rather than build more on top of it.

Here are some alternative ways we could handle this that I think are more principled:
* We could do `DenseMap<FileEntry *, FileEntryRef>` instead of the DenseSet, and similarly embed the FileEntryRef into the RHS type in cases with DenseMap
* Alternatively, we could add a wrapper type for FileEntryRef that continues to have the `FileEntry *` equality/hashing semantics, but where the name of that wrapper and/or the fact you need to unwrap it explicitly makes it obvious that the semantics are not "just" FileEntryRef.  If we can figure out a good name for this I think it would be a good solution.

https://github.com/llvm/llvm-project/pull/67742