Module: Use FileEntryRef and DirectoryEntryRef in Umbrella, Header, and DirectoryName, NFC

Push `FileEntryRef` and `DirectoryEntryRef` further, using it them
`Module::Umbrella`, `Module::Header::Entry`, and
`Module::DirectoryName::Entry`.

- Add `DirectoryEntryRef::operator const DirectoryEntry *` and
  `OptionalDirectoryEntryRefDegradesToDirectoryEntryPtr`, to get the
  same "degrades to `DirectoryEntry*` behaviour `FileEntryRef` enjoys
  (this avoids a bunch of churn in various clang tools).
- Fix the `DirectoryEntryRef` constructor from `MapEntry` to take it by
  `const&`.

Note that we cannot get rid of the `...AsWritten` names leveraging the
new classes, since these need to be as written in the `ModuleMap` file
and the module directory path is preprended for the lookup in the
`FileManager`.

Differential Revision: https://reviews.llvm.org/D90497