[clang][deps] Ensure deterministic filename case

The dependency scanner can reuse single FileManager instance across multiple translation units. This may lead to non-deterministic output depending on which TU gets processed first.

One of the problems is that Clang uses DirectoryEntry::getName in the header search algorithm. This function returns the path that was first used to construct the (shared) entry in FileManager. Using DirectoryEntryRef::getName instead preserves the case as it was spelled out for the current "get directory entry" request.

rdar://90647508

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D123229