Hotfix: Crash when parsing two object files

[fosterbrereton]

@bheath-adobe found a pair of object files that was causing ORC to crash. This set of changes fixes the crash.

Details of the changes:

• The compilation unit header offset is now passed to fetch_one_die. It was assuming the top of __debug_info sufficed for this value, which is true for the first compilation unit in the .o file. (Most .o files have a single compilation unit, so this "worked".) One of the sample files @bheath-adobe gave me has multiple compilation units in the .o file, and one of the successive ones was needed for ODRV processing. This caused fetch_one_die to crash trying to read from the top of __debug_info when it should have been reading relative to the successive compilation unit header when trying to resolve a type.
  • I also made improvements to distinguish between the offset to the compilation unit header and the offset to the compilation unit die, which are two related but separate things.
• refN attribute values are now calculated relative to the compilation unit header offset (which itself is relative to the top of __debug_info). This used to be a value calculated very strangely; the new calculation is exactly what the DWARF spec expects should happen.
• The internal terminology around variables named "address"/"offset" has been improved:
  • An ADDRESS is absolute relative to the top of the file. Address-based variables are always relative to the top of the file, so need no additional annotation.
  • An OFFSET is relative to either __debug_info or the start of the compilation unit (whose offset is relative to __debug_info.) Offsets should always be annotated with what their value is relative to.
  • Going forward, all DWARF/DIE/scanning related variables should follow the above conventions.
• A handful of improvements were made to some freader routines, courtesy of @bheath-adobe.
• Improvements made to self-referential type deduction (Honestly I'm still not entirely sure what this is.)

[leethomason]

LGTM. Surprisingly small fix.