Introduce a new key `generated_buffers`, which
stores an array of generated buffers. These
include the buffer text, as well as its original
location and any parent buffers.
While here, also fix rdar://107281079 such that
only apply the filename fallback logic to the
pretty-printed Decl case. We ought to remove this
fallback once the editor can handle it though.
rdar://107281079
rdar://107952288
