Verifier: Start recursing into !dbg attachments

The main verifier already recurses through the other entry points, so we
might as well descend here too.

This temporarily duplicates some work already done in
`verifyDebugInfo()`, but eventually I'll be removing the other side.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233095 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed