Verifier: Set --verify-debug-info=true by default

r186634 started verifying debug info, and r194986 disabled it by default
because it was too expensive to run the checks on every function (since
most of the graph was reachable from each function).

r206300 moved the checks to module-level to make it cheaper, but there
was already quite a bit of testcase bitrot (and the verifier would only
print `<badref>`) so I guess no one had time to turn it back on.

This does just that.  Upgrade scripts this past autumn and winter
probably fixed some of the bitrot, and this weekend I fixed the verifier
output (r232275, r232417, r232418) and thusly the remaining failing
testcases (r232290, r232415).

This is part of PR22777.

llvm-svn: 232505