ARCMigrate: Migrate ObjCMT.cpp over to FileEntryRef

Migrate ObjCMT.cpp from using `const FileEntry*` to `FileEntryRef`. This
is one of the blockers for changing `SourceManager` to use
`FileEntryRef`.

This adds an initial version of `SourceManager::getFileEntryRefForID`,
which uses to `FileEntry::getLastRef`; after `SourceManager` switches,
`SourceManager::getFileEntryForID` will need to call this function.

This also adds uses of `FileEntryRef` as a key in a `DenseMap`, and a
call to `hash_value(Optional)` in `DenseMapInfo<EditEntry>`; support for
these were added in prep commits.

Differential Revision: https://reviews.llvm.org/D92678