[clang] NFCI: Use DirectoryEntryRef in Module::Directory

This patch changes the type of `Module::Directory` from `const DirectoryEntry *` to (essentially) `Optional<DirectoryEntryRef>` in order to remove uses of the deprecated `DirectoryEntry::getName()`.

Depends on D127651.

Reviewed By: bnbarham

Differential Revision: https://reviews.llvm.org/D127654