Verifier: Check that @llvm.dbg.* intrinsics have a !dbg attachment

Before we start to rely on valid `!dbg` attachments, add a check to the
verifier that `@llvm.dbg.*` intrinsics always have one.  Also check that
the `scope:` fields point at the same `MDSubprogram`.

This is in the context of PR22778.  The check that the `inlinedAt:`
fields agree has baked for a while (since r234021), so I'll kill [1] the
`MDLocalVariable::getInlinedAt()` field soon.

[1]: http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150330/269387.html

Unfortunately, that means it's impossible to keep the current `Verifier`
checks, which rely on comparing `inlinedAt:` fields.  We'll be able to
keep the checks I'm adding here.

If this breaks your out-of-tree testcases, the upgrade script
(add-dbg-to-intrinsics.sh) attached to PR22778 that I used for r235040
might fix them for you.

llvm-svn: 235048

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