[ELF] --warn-backrefs: save the referenced InputFile *

For a diagnostic `A refers to B` where B refers to a bitcode file, if the
symbol gets optimized out, the user may see `A refers to <internal>`; if the
symbol is retained, the user may see `A refers to lto.tmp`.

Save the reference InputFile * in the DenseMap so that the original filename is
available in reportBackrefs().