Verifier: Remove the separate -verify-di pass

Remove `DebugInfoVerifierLegacyPass` and the `-verify-di` pass.
Instead, call into the `DebugInfoVerifier` from inside
`VerifierLegacyPass::finalizeModule()`.  This better matches the logic
in `verifyModule()` (used by the new PassManager), avoids requiring two
separate passes to verify the IR, and makes the API for "add a pass to
verify the IR" simple.

Note: the `-verify-debug-info` flag still works (for now, at least;
eventually it might make sense to just remove it).

llvm-svn: 232772