DebugInfo: emit the definition of types when construction vtables are required...

DebugInfo: emit the definition of types when construction vtables are required as these types may never end up emitting the full class data

This might be able to be optimized further by only doing this in the
absence of a key function, but it doesn't look like GCC is doing that so
I'm not rushing to do it just yet.

llvm-svn: 189022