VFS: Document goals of 'use-external-name' and related logic, NFC

Document 'use-external-name' and the various bits of logic that make it
work, to avoid others having to repeat the archival work (given that I
added getFileRefReturnsCorrectNameForDifferentStatPath to
FileManagerTest, seems possible I understood this once before!).

- b59cf679 added 'use-external-name' to
  RedirectingFileSystem. This causes `stat`s to return the external
  name for a redirected file instead of the name it was accessed by,
  leaking it through the VFS.
- d066d4c8 propagated the external name
  further through clang::FileManager.
- 4dc5573a, which added
  clang::FileEntryRef to clang::FileManager, has complicated concession
  to account for this as well (since refactored a bit).

The goal of 'use-external-name' is to enable Clang to report "real" file
paths to users (via diagnostics) and to external tools (such as
debuggers reading debug info and build systems reading `.d` files).

I've added FIXMEs to look at other channels for communicating the
external names, since the current implementation adds complexity to
FileManager and exposes an inconsistent interface to clients.

Besides that, the FileManager logic appears to be kicking in outside of
'use-external-name'. Seems that *some* vfs::FileSystem implementations
canonicalize some paths returned by `stat` in *some* cases (the bug
isn't fully understood yet). Volodymyr Sapsai is investigating, this at
least better documents what *is* understood.