Verifier: Explicitly verify type references

`verifyDebugInfo()` was doing two things:

  - Asserting on unresolved type references.
  - Calling `Verify()` functions for various types of debug info.

The `Verify()` functions have been gutted, so rename the function to
`verifyTypeRefs()` and explicitly check those.  Instead of assertions,
we get nice error messages now.

llvm-svn: 233664