[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