Revert "Revert "DebugInfo: Omit debug info for dynamic classes in TUs that do...

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.

llvm-svn: 188642