DebugInfo: Omit class definitions even in the presence of available_externally vtables

To ensure optimization level doesn't pessimize the -fstandalone-debug
vtable debug info optimization (where class definitions are only emitted
where the vtable is emitted - reducing redundant debug info) ensure the
debug info class definition is still omitted when an
available_externally vtable definition is emitted for optimization
purposes.

llvm-svn: 292768