[clang][modules] Adopt `FileEntryRef` in the `HeaderFileInfo` block in PCM files (#67383)

This patch adopts `FileEntryRef` in the `HeaderFileInfo`-writing part of
`ASTWriter`.

First, this patch removes the loop over
`FileManager::VirtualFileEntries`. It's redundant, since all virtual
file entries are also present in `SeenFileEntries` and thus already in
`UIDToFiles`.

Second, since we now no longer rely on
`FileEntry::getLastRef()`/`FileEntry::getName()`, this patch takes care
to establish which path gets used for each UID by picking the
`FileEntryRef` with the most "`<`" name (instead of just relying on the
`StringMap` iteration order).

Note that which `FileEntry`/`FileEntryRef` objects we pick for each UID
for serialization into the `llvm::OnDiskChainedHashTable` doesn't really
matter. The hash function only includes the file size and modification
time. The file name only plays role during resolution of hash
collisions, in which case it goes through `FileManager` and resolves to
a `FileEntry` that gets pointer-compared with the queried `FileEntry`.

(Reincarnation of [D143414](https://reviews.llvm.org/D143414) and
[D142780](https://reviews.llvm.org/D142780).)

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