[clang][lex] NFCI: Use DirectoryEntryRef in HeaderSearch::load*()

This patch removes uses of the deprecated `DirectoryEntry::getName()` from `HeaderSearch::load*()` functions by using `DirectoryEntryRef` instead.

Note that we bail out in one case and use the also deprecated `FileEntry::getLastRef()`. That's to prevent this patch from growing, and is addressed in a follow-up.

Reviewed By: bnbarham

Differential Revision: https://reviews.llvm.org/D123771