Check optional/expected/required attributes

Check optional/expected/required attributes

* The code currently iterates over tree of DIEs.  This will be necessary
  for checking parent vs. expected children, but for attributes, it's an
  overkill, and we should check abbreviation table instead.  Also, this
  way, we report each mistake in abbreviation table separately for each
  DIE that uses the erroneous entry.

* All attributes are marked optional.  In future, we need to go through
  the standard, or employ some other source of knowledge (hi, Roland!),
  and adjust the optionality level.

* Vast majority of entries are derived from the table in Appendix A of the
  standard.  Some were added to reflect what I found in several debuginfo
  test cases that I've tried to check.  These entries are marked with
  "XXX" comment to distinguish them from the rest.