ObjC IVars aren't added when the type isn't emitted after the interface extension
Build debug info for ObjC interface types at the end of the translation unit to ensure all ivars are included.

This takes a different approach than the
completedType/requiresCompleteType work which relies on AST callbacks to
upgrade the type declaration to a definition. Instead, just defer
constructing the definition to the end of the translation unit.

This works because the definition is never needed by other debug info
(so far as I know), whereas the definition of a struct may be needed by
other debug info before the end of the translation unit (such as
emitting the definition of a member function which must refer to that
member function's declaration).

If we had a callback for whenever an IVar was added to an ObjC interface
we could use that, and remove the need for the ObjCInterfaceCache, which
might be nice. (also would need a callback for when it was more than
just a declaration so we could get properties, etc).

A side benefit is that we also don't need the CompletedTypeCache
anymore. Just rely on the declaration-ness of a type to decide whether
its definition is yet to be emitted.

There's still the PR19562 memory leak, but this should hopefully make
that a bit easier to approach.