Adds is_paramlist() to t_struct.
Adds is_paramlist() to t_struct.

Summary: Adds the `is_paramlist()` getter method to `t_struct`. Also intiailizes the fields to avoid potential uninitialized memory access.

Reviewed By: yfeldblum

Differential Revision: D9466481

fbshipit-source-id: 51c3c67062ab63f23b2df22e0e37ab36689919ab

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