Revert "Revert "DebugInfo: Omit debug info for dynamic classes in TUs that do not have the vtable for that class""

This reverts commit r188600.

r188640/r188639 fixed the root cause of the crash-on-valid that r188600
originally introduced. This now appears to bootstrap debug clang
successfully to the best of my testing.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188642 91177308-0d34-0410-b5e6-96231b3b80d8