Final separate debug info patch

[Jim Blandy]

Some other stuff I've noticed:

- In reread_symbols, it looks like we do throw away
  objfile->separate_debug_objfile if it doesn't match any more, but I
  don't see where we read in the new one.  (I think this is the cause
  of the reread.exp failure.)

- Similarly, if we start with an unseparated objfile, but then it is
  changed to a separated objfile, I don't see where reread_symbols
  loads the separated objfile's debug objfile.

- In reread_symbols, if the new objfile's debug link has a different
  name, but the CRC's happen to match, it won't toss the old file.  It
  seems to me that we should re-run find_separate_debug_file from
  scratch, and make sure the filename we get matches the existing
  separate_debug_objfile's name.

In general, I'm not convinced that the change to reread_symbols really
covers all the cases.  I'll try to fix it.