SourceManager: Migrate to FileEntryRef in getOrCreateContentCache, NFC

Change `SourceManager::getOrCreateContentCache` to take a `FileEntryRef`
and update call sites (mostly internal to SourceManager.cpp). In a
couple of cases this temporarily relies on `FileEntry::getLastRef`, but
those can be cleaned up once other APIs switch over.

The one change outside of SourceManager.cpp is in ASTReader.cpp, which
stops relying on the auto-degrade-to-`FileEntry*` behaviour from
`InputFile::getFile` since it now needs a `FileEntryRef`.

No functionality change here.

Differential Revision: https://reviews.llvm.org/D92983