[clang][lex] NFCI: Use DirectoryEntryRef in ModuleMap::parseModuleMapFile()

This patch changes the argument type of `ModuleMap::parseModuleMapFile()` from `const DirectoryEntry *` to `DirectoryEntryRef` in order to remove the deprecated uses of `DirectoryEntry::getName()`.

Depends on D127648.

Reviewed By: bnbarham

Differential Revision: https://reviews.llvm.org/D127651