DebugInfo: Contrain the record type parameter for CollectRecordFields

This is the correct type (as is demonstrated by the fact that the caller
didn't need to change) & will be useful in a future patch.
DebugInfo: Omit debug info for dynamic classes in TUs that do not have the vtable for that class

This reduces Clang's .dwo (fission debug info) size by 23% over
Clang+LLVM.