SourceManager: Change SourceManager::isMainFile to take a FileEntry, NFC

`SourceManager::isMainFile` does not use the filename, so it doesn't
need the full `FileEntryRef`; in fact, it's misleading to take the name
because that makes it look relevant. Simplify the API, and in the
process remove some calls to `FileEntryRef::FileEntryRef` in the unit
tests (which were blocking making that private to `SourceManager`).

Differential Revision: https://reviews.llvm.org/D89507