Fix crasher in CodeView test.

Apparently this was always broken, but previously we were more
graceful about it and we would print "unknown udt" if we couldn't
find the type index, whereas now we just segfault because we
assume it's valid.  But this exposed a real bug, which is that
we weren't looking in the right place.  So fix that, and also
fix this crash at the same time.

llvm-svn: 303397