[clang][lex] NFCI: Use FileEntryRef in ModuleMap::{load,lookup}ModuleMap()

This patch changes the return/argument types of `ModuleMap::{load,lookup}ModuleMap()` from `const FileEntry *` to `FileEntryRef` in order to remove uses of the deprecated `DirectoryEntry::getName()`.

Reviewed By: bnbarham

Differential Revision: https://reviews.llvm.org/D127647