Basic: Change Module::Umbrella to a PointerUnion, NFC

Change `Module::Umbrella` from a `const void *` to a `PointerUnion` of
`FileEntry` and `DirectoryEntry`. We can drop the `HasUmbrellaDir` bit
(since `PointerUnion` includes that).

This change makes it safer to update to `FileEntryRef` and
`DirectoryEntryRef` in a future patch.

Differential Revision: https://reviews.llvm.org/D90481