[clang][lex] NFCI: Use DirectoryEntryRef in FrameworkCacheEntry

This patch changes the member of `FrameworkCacheEntry` from `const DirectoryEntry *` to `Optional<DirectoryEntryRef>` in order to remove uses of the deprecated `DirectoryEntry::getName()`.

Reviewed By: bnbarham

Differential Revision: https://reviews.llvm.org/D123854