[clang] Implement `PointerLikeTraits` for `{File,Directory}EntryRef`

This patch implements `llvm::PointerLikeTraits<FileEntryRef>` and `llvm::PointerLikeTraits<DirectoryEntryRef>`, allowing some simplifications around umbrella header/directory code.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D154905