DWARFVerifier: Don't parse all units twice

Introduced/discussed in https://reviews.llvm.org/D38719

The header validation logic was also explicitly building the DWARFUnits
to validate. But then other calls, like "Units.getUnitForOffset" creates
the DWARFUnits again in the DWARFContext proper - so, let's avoid
creating the DWARFUnits twice by walking the DWARFContext's units rather
than building a new list explicitly.

This does reduce some verifier power - it means that any unit with a
header parsing failure won't get further validation, whereas the
verifier-created units were getting some further validation despite
invalid headers. I don't think this is a great loss/seems "right" in
some ways to me that if the header's invalid we should stop there.

Exposing the raw DWARFUnitVectors from DWARFContext feels a bit
sub-optimal, but gave simple access to the getUnitForOffset to keep the
rest of the code fairly similar.