Verifier: Move over DISubprogram::Verify()

Move over the remaining (somewhat complicated) check from
`DISubprogram::Verify()`.  I suspect this check could be optimized --
e.g., it would be nice not to do another full traversal here -- but it's
not exactly obvious how.  For now, just bring it over as is.

Once we have a better model for the "canonical" subprogram of a
`Function`, we should enforce that all `!dbg` attachments lead to the
canonical one.

llvm-svn: 233663