[clangd] Don't ignore external HFI in `SymbolCollector` (#88446)

The `FileEntry` corresponds to a `FileID` containing the
`SourceLocation` of a `NamedDecl` which (I think) might've been
deserialized from a PCM file. Considering external `HeaderFileInfo` here
is most likely the right thing to do here in order to get the correct
spelling in case the current compiler instance has not register this
file as a header yet.