[clangd] Avoid expensive checks of buffer names in IncludeCleaner

This changes the handling of special buffers (<command-line> etc) that
SourceManager treats as files but FileManager does not.

We now include them in findReferencedFiles() and drop them as part of
translateToHeaderIDs(). This pairs more naturally with the data representations
we're using, and so avoids a bunch of converting between representations for
filtering.

Differential Revision: https://reviews.llvm.org/D112652