FileManager: Shrink FileEntryRef to the size of a pointer
Shrink `FileEntryRef` to the size of a pointer, by having it directly
reference the `StringMapEntry` the same way that `DirectoryEntryRef`
does. This makes `FileEntryRef::FileEntryRef` private as a side effect
(`FileManager` is a friend!).
There are two helper types added within `FileEntryRef`:
- `FileEntryRef::MapValue` is the type stored in
`FileManager::SeenFileEntries`. It's a replacement for
`SeenFileEntryOrRedirect`, where the second pointer type has been
changed from `StringRef*` to `MapEntry*` (see next bullet).
- `FileEntryRef::MapEntry` is the instantiation of `StringMapEntry<>`
where `MapValue` is stored. This is what `FileEntryRef` has a pointer
to, in order to grab the name in addition to the value.
Differential Revision: https://reviews.llvm.org/D89488