[clang][index] NFCI: Make `CXFile` a `FileEntryRef`

This patch swaps out the `void *` behind `CXFile` from `FileEntry *` to `FileEntryRef::MapEntry *`. This allows us to remove some deprecated uses of `FileEntry::getName()`.

Depends on D151854.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D151938