[hack] Do not attempt to specialize TypeStructure's type for unbound abstract types
[hack] Do not attempt to specialize TypeStructure's type for unbound abstract types

Summary: Originally in {D2534747} I didn't specialize unbound generics since we do not
know exactly what they will be at run time so we leave all fields as nullable.
We should do the same for all abstract kinds (expect enums).

Reviewed By: dabek

Differential Revision: D2552452

fb-gh-sync-id: 7885423b9f4f2899e265c3bac0640fda161c3484

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