tbv2: codegen fails on top level incomplete objects

[JakeHillion]

CodeGen-v2 doesn't have enough information to generate TreeBuilder information for a top level incomplete structure (struct IncompleteStruct*). Whether we should care about this is another question... Running OIL directly on an incomplete structure is entirely pointless.

Examples:

• OilIntegration.pointers_incomplete_raw
• OilIntegration.pointers_incomplete_raw_no_follow
• OilIntegration.pointers_incomplete_raw_null