[clang][lex] NFCI: Use FileEntryRef in PPCallbacks::InclusionDirective()

[clang][lex] NFCI: Use FileEntryRef in PPCallbacks::InclusionDirective()

This patch changes type of the `File` parameter in `PPCallbacks::InclusionDirective()` from `const FileEntry *` to `Optional<FileEntryRef>`.

With the API change in place, this patch then removes some uses of the deprecated `FileEntry::getName()` (e.g. in `DependencyGraph.cpp` and `ModuleDependencyCollector.cpp`).

Reviewed By: dexonsmith, bnbarham

Differential Revision: https://reviews.llvm.org/D123574