It can now:
- not validate (=none)
- validate that all symbols in the IR are also in the TBD (=missing),
- validate the above, and also that all in the TBD are in the IR (=all).
The first and last were switched between with the old boolean flag, the
second is new.
Only one of these tests actually "passes", since TBDGen is incomplete,
but the use of FileCheck means they'll be flagged to be updated as
TBDGen improves: the failures are known problems.
