Verifier: Check that !dbg attachments have the right type

A WIP patch makes `DIDescriptor` accessors more strict, which in turn
causes the `DebugInfoFinder` to crash on wrongly typed `!dbg`
attachments.  Catch that error up front in
`Verifier::visitInstruction()`.

Also remove a test that we "handle" invalid `!dbg` attachments, added
back in r99938.  We don't want to handle those anymore.

Note: I'm *not* recursing and verifying the debug info graph reachable
from this node; that work is already done by `verifyDebugInfo()`.

llvm-svn: 232834