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

[Ben Langmuir via cfe-commits]

benlangmuir wrote:

For what it's worth, until recently there was almost no **use** of the implicit unique file semantics for FileEntryRef even though the DenseMapInfo had been available for a long time.  But okay, I see there are more uses now so this is not the right place to push back on that.

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