FileManager: Add FileEntryRef::getDir, returning DirectoryEntryRef

Add `FileEntryRef::getDir`, which returns a `DirectoryEntryRef`. This
includes a few changes:

- Customize `OptionalStorage` so that `Optional<DirectoryEntryRef>` is
  pointer-sized (like the change made to `Optional<FileEntryRef>`).
  Factored out a common class, `FileMgr::MapEntryOptionalStorage`, to
  reduce the code duplication.
- Store an `Optional<DirectoryEntryRef>` in `FileEntryRef::MapValue`.
  This is set if and only if `MapValue` has a real `FileEntry`.
- Change `FileManager::getFileRef` and `getVirtualFileRef` to use
  `getDirectoryRef` and store it in the `StringMap` for `FileEntryRef`.

Differential Revision: https://reviews.llvm.org/D90484