swift234

IRGen: Stub out some dependencies on fixed layout in class metadata codegen.

Some things we'll need to arrange to instantiate at runtime:

- The deallocation size in the deallocating destructor,
- The ivar offset pointers for dependent-layout fields,
- The ivar start offset in the rodata.

For now, just fill in zero as a placeholder until the runtime pieces are in place so the compiler doesn't assert out when emitting generic classes.

Swift SVN r9195